diff --git a/backend/app/cmd/cmd.go b/backend/app/cmd/cmd.go
index 4d257192..534c1788 100644
--- a/backend/app/cmd/cmd.go
+++ b/backend/app/cmd/cmd.go
@@ -22,6 +22,7 @@ type CommonOptionsCommander interface {
 	SetCommon(commonOpts CommonOpts)
 	Execute(args []string) error
 	HandleDeprecatedFlags() []DeprecatedFlag
+	FindDeprecatedFlagsCollisions() []DeprecatedFlag
 }
 
 // CommonOpts sets externally from main, shared across all commands
@@ -49,6 +50,10 @@ func (c *CommonOpts) SetCommon(commonOpts CommonOpts) {
 // HandleDeprecatedFlags sets new flags from deprecated and returns their list
 func (c *CommonOpts) HandleDeprecatedFlags() []DeprecatedFlag { return nil }
 
+// FindDeprecatedFlagsCollisions returns list of flags collisions, e.g. when both deprecated
+// and new flags are set to non-default values, and different from each other.
+func (c *CommonOpts) FindDeprecatedFlagsCollisions() []DeprecatedFlag { return nil }
+
 // fileParser used to convert template strings like blah-{{.SITE}}-{{.YYYYMMDD}} the final format
 type fileParser struct {
 	site string
diff --git a/backend/app/cmd/server.go b/backend/app/cmd/server.go
index 89b2b919..06ebcee1 100644
--- a/backend/app/cmd/server.go
+++ b/backend/app/cmd/server.go
@@ -321,7 +321,9 @@ func (s *ServerCommand) Execute(_ []string) error {
 	return nil
 }
 
-// HandleDeprecatedFlags sets new flags from deprecated returns their list
+// HandleDeprecatedFlags sets new flags from deprecated returns their list.
+// Returned list has DeprecatedFlag.Old and DeprecatedFlag.Version set, and DeprecatedFlag.New is optional
+// (as some entries are removed without substitute).
 func (s *ServerCommand) HandleDeprecatedFlags() (result []DeprecatedFlag) {
 	if s.Auth.Email.Host != "" && s.SMTP.Host == "" {
 		s.SMTP.Host = s.Auth.Email.Host
@@ -343,7 +345,8 @@ func (s *ServerCommand) HandleDeprecatedFlags() (result []DeprecatedFlag) {
 		s.SMTP.Password = s.Auth.Email.SMTPPassword
 		result = append(result, DeprecatedFlag{Old: "auth.email.passwd", New: "smtp.password", Version: "1.5"})
 	}
-	if s.Auth.Email.TimeOut != 10*time.Second && s.SMTP.TimeOut == 10*time.Second {
+	const emailDefaultTimout = 10 * time.Second
+	if s.Auth.Email.TimeOut != emailDefaultTimout && s.SMTP.TimeOut == emailDefaultTimout {
 		s.SMTP.TimeOut = s.Auth.Email.TimeOut
 		result = append(result, DeprecatedFlag{Old: "auth.email.timeout", New: "smtp.timeout", Version: "1.5"})
 	}
@@ -368,8 +371,8 @@ func (s *ServerCommand) HandleDeprecatedFlags() (result []DeprecatedFlag) {
 		s.Telegram.Token = s.Notify.Telegram.Token
 		result = append(result, DeprecatedFlag{Old: "notify.telegram.token", New: "telegram.token", Version: "1.9"})
 	}
-	const telegramDefaultDuration = time.Second * 5
-	if s.Notify.Telegram.Timeout != telegramDefaultDuration && s.Telegram.Timeout == telegramDefaultDuration {
+	const telegramDefaultTimeout = time.Second * 5
+	if s.Notify.Telegram.Timeout != telegramDefaultTimeout && s.Telegram.Timeout == telegramDefaultTimeout {
 		s.Telegram.Timeout = s.Notify.Telegram.Timeout
 		result = append(result, DeprecatedFlag{Old: "notify.telegram.timeout", New: "telegram.timeout", Version: "1.9"})
 	}
@@ -379,6 +382,40 @@ func (s *ServerCommand) HandleDeprecatedFlags() (result []DeprecatedFlag) {
 	return result
 }
 
+// FindDeprecatedFlagsCollisions returns flags which are set both old (deprecated) and new way,
+// which means new ones are used and old ones are ignored by deprecated flag handler.
+// It returns DeprecatedFlag list which always has only DeprecatedFlag.Old and DeprecatedFlag.New set.
+func (s *ServerCommand) FindDeprecatedFlagsCollisions() (result []DeprecatedFlag) {
+	if stringsSetAndDifferent(s.Auth.Email.Host, s.SMTP.Host) {
+		result = append(result, DeprecatedFlag{Old: "auth.email.host", New: "smtp.host"})
+	}
+	if s.Auth.Email.Port != 0 && s.SMTP.Port != 0 && s.Auth.Email.Port != s.SMTP.Port {
+		result = append(result, DeprecatedFlag{Old: "auth.email.port", New: "smtp.port"})
+	}
+	if stringsSetAndDifferent(s.Auth.Email.SMTPUserName, s.SMTP.Username) {
+		result = append(result, DeprecatedFlag{Old: "auth.email.user", New: "smtp.username"})
+	}
+	if stringsSetAndDifferent(s.Auth.Email.SMTPPassword, s.SMTP.Password) {
+		result = append(result, DeprecatedFlag{Old: "auth.email.passwd", New: "smtp.password"})
+	}
+	const emailDefaultTimout = 10 * time.Second
+	if s.Auth.Email.TimeOut != emailDefaultTimout && s.SMTP.TimeOut != emailDefaultTimout && s.Auth.Email.TimeOut != s.SMTP.TimeOut {
+		result = append(result, DeprecatedFlag{Old: "auth.email.timeout", New: "smtp.timeout"})
+	}
+	if !(len(s.Notify.Type) == 1 && contains("none", s.Notify.Type)) && // default, "none" notify type
+		(len(s.Notify.Users) != 0 || len(s.Notify.Admins) != 0) { // new notify param(s) are used, old ones will be ignored
+		result = append(result, DeprecatedFlag{Old: "notify.type", New: "notify.(users|admins)"})
+	}
+	if stringsSetAndDifferent(s.Notify.Telegram.Token, s.Telegram.Token) {
+		result = append(result, DeprecatedFlag{Old: "notify.telegram.token", New: "telegram.token"})
+	}
+	const telegramDefaultTimeout = time.Second * 5
+	if s.Notify.Telegram.Timeout != telegramDefaultTimeout && s.Telegram.Timeout != telegramDefaultTimeout && s.Notify.Telegram.Timeout != s.Telegram.Timeout {
+		result = append(result, DeprecatedFlag{Old: "notify.telegram.timeout", New: "telegram.timeout"})
+	}
+	return result
+}
+
 func (s *ServerCommand) handleDeprecatedNotifications() {
 	for _, t := range s.Notify.Type {
 		if t == "email" && !contains(t, s.Notify.Users) {
@@ -390,6 +427,13 @@ func (s *ServerCommand) handleDeprecatedNotifications() {
 	}
 }
 
+func stringsSetAndDifferent(s1, s2 string) bool {
+	if s1 != "" && s2 != "" && s1 != s2 {
+		return true
+	}
+	return false
+}
+
 func contains(s string, a []string) bool {
 	for _, t := range a {
 		if t == s {
diff --git a/backend/app/cmd/server_test.go b/backend/app/cmd/server_test.go
index 074dce1b..6437263f 100644
--- a/backend/app/cmd/server_test.go
+++ b/backend/app/cmd/server_test.go
@@ -410,7 +410,10 @@ func TestServerApp_DeprecatedArgs(t *testing.T) {
 	p := flags.NewParser(&s, flags.Default)
 	args := []string{
 		"test",
+		"--notify.type=email",
 		"--notify.type=telegram",
+		"--img-proxy",
+		"--notify.email.notify_admin",
 		"--auth.email.host=smtp.example.org",
 		"--auth.email.port=666",
 		"--auth.email.tls",
@@ -420,6 +423,7 @@ func TestServerApp_DeprecatedArgs(t *testing.T) {
 		"--auth.email.template=file.tmpl",
 		"--notify.telegram.token=abcd",
 		"--notify.telegram.timeout=3m",
+		"--notify.telegram.api=http://example.org",
 	}
 	assert.Empty(t, s.SMTP.Host)
 	assert.Empty(t, s.SMTP.Port)
@@ -439,9 +443,12 @@ func TestServerApp_DeprecatedArgs(t *testing.T) {
 			{Old: "auth.email.passwd", New: "smtp.password", Version: "1.5"},
 			{Old: "auth.email.timeout", New: "smtp.timeout", Version: "1.5"},
 			{Old: "auth.email.template", Version: "1.5"},
+			{Old: "img-proxy", New: "image-proxy.http2https", Version: "1.5"},
+			{Old: "notify.email.notify_admin", New: "notify.admins=email", Version: "1.9"},
 			{Old: "notify.type", New: "notify.(users|admins)", Version: "1.9"},
 			{Old: "notify.telegram.token", New: "telegram.token", Version: "1.9"},
 			{Old: "notify.telegram.timeout", New: "telegram.timeout", Version: "1.9"},
+			{Old: "notify.telegram.api", Version: "1.9"},
 		},
 		deprecatedFlags)
 	assert.Equal(t, "smtp.example.org", s.SMTP.Host)
@@ -452,6 +459,61 @@ func TestServerApp_DeprecatedArgs(t *testing.T) {
 	assert.Equal(t, 15*time.Second, s.SMTP.TimeOut)
 }
 
+func TestServerApp_DeprecatedArgsCollisions(t *testing.T) {
+	s := ServerCommand{}
+	s.SetCommon(CommonOpts{RemarkURL: "https://demo.remark42.com", SharedSecret: "123456"})
+
+	p := flags.NewParser(&s, flags.Default)
+	args := []string{
+		"test",
+		"--auth.email.host=smtp-old.example.org",
+		"--smtp.host=smtp-new.example.org",
+		"--auth.email.port=666",
+		"--smtp.port=999",
+		"--auth.email.user=test_user",
+		"--smtp.username=new_test_user",
+		"--auth.email.passwd=test_password",
+		"--smtp.password=new_test_password",
+		"--auth.email.timeout=15s",
+		"--smtp.timeout=20s",
+		"--notify.type=telegram",
+		"--notify.users=telegram",
+		"--notify.telegram.token=abcd",
+		"--telegram.token=dcba",
+		"--notify.telegram.timeout=3m",
+		"--telegram.timeout=5m",
+	}
+	_, err := p.ParseArgs(args)
+	require.NoError(t, err)
+	deprecatedFlagsCollisions := s.FindDeprecatedFlagsCollisions()
+	assert.ElementsMatch(t,
+		[]DeprecatedFlag{
+			{Old: "notify.type", New: "notify.(users|admins)"},
+			{Old: "auth.email.host", New: "smtp.host"},
+			{Old: "auth.email.port", New: "smtp.port"},
+			{Old: "auth.email.user", New: "smtp.username"},
+			{Old: "auth.email.passwd", New: "smtp.password"},
+			{Old: "auth.email.timeout", New: "smtp.timeout"},
+			{Old: "notify.telegram.token", New: "telegram.token"},
+			{Old: "notify.telegram.timeout", New: "telegram.timeout"},
+		},
+		deprecatedFlagsCollisions)
+
+	// case which should return nothing
+	s = ServerCommand{}
+	s.SetCommon(CommonOpts{RemarkURL: "https://demo.remark42.com", SharedSecret: "123456"})
+	p = flags.NewParser(&s, flags.Default)
+	args = []string{
+		"test",
+		"--auth.email.host=smtp-old.example.org",
+		"--smtp.host=''",
+	}
+	_, err = p.ParseArgs(args)
+	require.NoError(t, err)
+	deprecatedFlagsCollisions = s.FindDeprecatedFlagsCollisions()
+	assert.Empty(t, []DeprecatedFlag{}, deprecatedFlagsCollisions)
+}
+
 func Test_ACMEEmail(t *testing.T) {
 	cmd := ServerCommand{}
 	cmd.SetCommon(CommonOpts{RemarkURL: "https://remark.com:443", SharedSecret: "123456"})
diff --git a/backend/app/main.go b/backend/app/main.go
index 2371349c..77eeac9f 100644
--- a/backend/app/main.go
+++ b/backend/app/main.go
@@ -45,6 +45,9 @@ func main() {
 			SharedSecret: opts.SharedSecret,
 			Revision:     revision,
 		})
+		for _, entry := range c.FindDeprecatedFlagsCollisions() {
+			log.Print(fmt.Sprintf("[ERROR] deprecated --%s and new --%s options are set to different values, old one is ignored: please remove it", entry.Old, entry.New))
+		}
 		for _, entry := range c.HandleDeprecatedFlags() {
 			deprecationNote := fmt.Sprintf("[WARN] --%s is deprecated since v%s and will be removed in the future", entry.Old, entry.Version)
 			if entry.New != "" {