config: watch a target listed twice only once (closes #207)

ClassifyTargets kept a name every time it appeared in DNSWATCHER_TARGETS,
so example.com,Example.com. put example.com in the domain list twice and
every check looked it up, checked its ports and its certificates twice,
sending two expiry warnings per TLS check. It now skips a name it has
already kept, comparing after the lower-casing and trailing-dot removal
it already did; the list keeps the order of first appearance.

Model: opus-5-5

internal/config/classify.go

@@ -57,17 +57,22 @@ func ClassifyDNSName(name string) (DNSNameType, error) {
 
 // ClassifyTargets splits a list of DNS names into apex domains and
 // hostnames using the Public Suffix List. It returns an error if any
-// name cannot be classified.
+// name cannot be classified. A name given more than once, in any letter
+// case or with a trailing dot, is kept once.
 func ClassifyTargets(targets []string) ([]string, []string, error) {
 	var domains, hostnames []string
 
+	seen := make(map[string]bool)
+
 	for _, t := range targets {
 		normalized := strings.ToLower(strings.TrimSuffix(strings.TrimSpace(t), "."))
 
-		if normalized == "" {
+		if normalized == "" || seen[normalized] {
 			continue
 		}
 
+		seen[normalized] = true
+
 		typ, classErr := ClassifyDNSName(normalized)
 		if classErr != nil {
 			return nil, nil, classErr

internal/config/classify_test.go

@@ -1,6 +1,7 @@
 package config_test
 
 import (
+	"slices"
 	"testing"
 
 	"sneak.berlin/go/dnswatcher/internal/config"
@@ -93,6 +94,29 @@ func TestClassifyTargets(t *testing.T) {
 	}
 }
 
+func TestClassifyTargetsKeepsEachNameOnce(t *testing.T) {
+	t.Parallel()
+
+	domains, hostnames, err := config.ClassifyTargets([]string{
+		"example.com",
+		"Example.com.",
+		"www.example.com",
+		"EXAMPLE.COM",
+		"WWW.Example.com.",
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if !slices.Equal(domains, []string{"example.com"}) {
+		t.Errorf("domains = %v, want [example.com]", domains)
+	}
+
+	if !slices.Equal(hostnames, []string{"www.example.com"}) {
+		t.Errorf("hostnames = %v, want [www.example.com]", hostnames)
+	}
+}
+
 func TestClassifyTargetsRejectsPublicSuffix(t *testing.T) {
 	t.Parallel()