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