cmd/age: warn about duplicate command-line arguments

Fixes #284

cmd/age/age.go

@@ -10,10 +10,12 @@ import (
 	"flag"
 	"fmt"
 	"io"
+	"iter"
 	"os"
 	"path/filepath"
 	"regexp"
 	"runtime/debug"
+	"slices"
 	"strings"
 
 	"filippo.io/age"
@@ -215,6 +217,16 @@ func main() {
 		}
 	}
 
+	warnDuplicates(slices.Values(recipientFlags), "recipient")
+	warnDuplicates(slices.Values(recipientsFileFlags), "recipients file")
+	warnDuplicates(func(yield func(string) bool) {
+		for _, f := range identityFlags {
+			if f.Type == "i" && !yield(f.Value) {
+				return
+			}
+		}
+	}, "identity file")
+
 	var inUseFiles []string
 	for _, i := range identityFlags {
 		if i.Type != "i" {
@@ -550,3 +562,15 @@ func absPath(name string) string {
 	}
 	return name
 }
+
+func warnDuplicates(s iter.Seq[string], name string) {
+	seen := make(map[string]bool)
+	warned := make(map[string]bool)
+	for e := range s {
+		if seen[e] && !warned[e] {
+			warningf("duplicate %s %q", name, e)
+			warned[e] = true
+		}
+		seen[e] = true
+	}
+}

cmd/age/testdata/duplicates.txt

@@ -0,0 +1,49 @@
+# Test duplicate recipient detection
+age -r age1jh0u9yam5jhql7vy4acz90p0rhqcr0rcyu4zm85s9sdcwrs7rfgshtdg85 -r age1yv2sxrjnyymx4qkn3ehdd5zdq4xnrea5ljm4hsmgxn5tpv985anqkrwqa7 -r age1jh0u9yam5jhql7vy4acz90p0rhqcr0rcyu4zm85s9sdcwrs7rfgshtdg85 -o test.age input
+stderr 'warning: duplicate recipient "age1jh0u9yam5jhql7vy4acz90p0rhqcr0rcyu4zm85s9sdcwrs7rfgshtdg85"'
+
+# Test duplicates separated by different argument
+age -r age1jh0u9yam5jhql7vy4acz90p0rhqcr0rcyu4zm85s9sdcwrs7rfgshtdg85 -a -r age1jh0u9yam5jhql7vy4acz90p0rhqcr0rcyu4zm85s9sdcwrs7rfgshtdg85 -o test2.age input
+stderr 'warning: duplicate recipient "age1jh0u9yam5jhql7vy4acz90p0rhqcr0rcyu4zm85s9sdcwrs7rfgshtdg85"'
+
+# Test duplicate recipients file detection
+age -R recipients1.txt -R recipients2.txt -R recipients1.txt -o test3.age input
+stderr 'warning: duplicate recipients file "recipients1.txt"'
+
+# Test duplicates separated by output flag
+age -R recipients1.txt -o test4.age -R recipients1.txt input
+stderr 'warning: duplicate recipients file "recipients1.txt"'
+
+# First create an encrypted file for decrypt tests
+age -r age1jh0u9yam5jhql7vy4acz90p0rhqcr0rcyu4zm85s9sdcwrs7rfgshtdg85 -o encrypted.age input
+
+# Test duplicate identity file detection (decrypt mode)
+age -d -i key1.txt -i key2.txt -i key1.txt encrypted.age
+stderr 'warning: duplicate identity file "key1.txt"'
+
+# Test duplicates separated by different argument in decrypt mode
+age -d -i key1.txt -o test.out -i key1.txt encrypted.age
+stderr 'warning: duplicate identity file "key1.txt"'
+
+# Test no warning when no duplicates
+age -r age1jh0u9yam5jhql7vy4acz90p0rhqcr0rcyu4zm85s9sdcwrs7rfgshtdg85 -r age1yv2sxrjnyymx4qkn3ehdd5zdq4xnrea5ljm4hsmgxn5tpv985anqkrwqa7 -o test5.age input
+! stderr 'warning: duplicate'
+
+# Test multiple duplicates (same value repeated 3+ times)
+age -r age1jh0u9yam5jhql7vy4acz90p0rhqcr0rcyu4zm85s9sdcwrs7rfgshtdg85 -r age1jh0u9yam5jhql7vy4acz90p0rhqcr0rcyu4zm85s9sdcwrs7rfgshtdg85 -r age1jh0u9yam5jhql7vy4acz90p0rhqcr0rcyu4zm85s9sdcwrs7rfgshtdg85 -o test6.age input
+stderr 'warning: duplicate recipient "age1jh0u9yam5jhql7vy4acz90p0rhqcr0rcyu4zm85s9sdcwrs7rfgshtdg85"'
+
+-- input --
+test data
+-- recipients1.txt --
+age1jh0u9yam5jhql7vy4acz90p0rhqcr0rcyu4zm85s9sdcwrs7rfgshtdg85
+-- recipients2.txt --
+age1yv2sxrjnyymx4qkn3ehdd5zdq4xnrea5ljm4hsmgxn5tpv985anqkrwqa7
+-- key1.txt --
+# created: 2025-12-22T22:06:22+01:00
+# public key: age1jh0u9yam5jhql7vy4acz90p0rhqcr0rcyu4zm85s9sdcwrs7rfgshtdg85
+AGE-SECRET-KEY-1WRM2S8SP3XSKLLXAXS489EXZNKCKRZWYQLQ8D2NRNQWCVAPSMA9SC5JWZQ
+-- key2.txt --
+# created: 2025-12-22T22:06:27+01:00
+# public key: age1yv2sxrjnyymx4qkn3ehdd5zdq4xnrea5ljm4hsmgxn5tpv985anqkrwqa7
+AGE-SECRET-KEY-1WZ3MRPAWEWR4DG474H460MXX7J2T0TEYNJ0SKQDMKP02JU7UJ9UQFGLZCE