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 and checked its certificates twice, sending two
expiry warnings per TLS check (port checks were already grouped by address
and port). 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.org",
+		"Example.org.",
+		"www.example.org",
+		"EXAMPLE.ORG",
+		"WWW.Example.org.",
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if !slices.Equal(domains, []string{"example.org"}) {
+		t.Errorf("domains = %v, want [example.org]", domains)
+	}
+
+	if !slices.Equal(hostnames, []string{"www.example.org"}) {
+		t.Errorf("hostnames = %v, want [www.example.org]", hostnames)
+	}
+}
+
 func TestClassifyTargetsRejectsPublicSuffix(t *testing.T) {
 	t.Parallel()