all: fix client duplicate uids

CHANGELOG.md

@@ -34,6 +34,8 @@ NOTE: Add new changes BELOW THIS COMMENT.
 
 ### Fixed
 
+- Acceptance of duplicate UIDs for persistent clients at startup.  See also a
+  section on client settings on the [Wiki page][wiki-config].
 - Issues with QUIC and HTTP/3 upstreams on older Linux kernel versions
   ([#6422]).
 - YouTube restricted mode is not enforced by HTTPS queries on Firefox.

internal/client/index.go

@@ -85,10 +85,16 @@ func (ci *Index) Add(c *Persistent) {
 // Clashes returns an error if the index contains a different persistent client
 // with at least a single identifier contained by c.  c must be non-nil.
 func (ci *Index) Clashes(c *Persistent) (err error) {
+	p, ok := ci.uidToClient[c.UID]
+	if ok {
+		return fmt.Errorf("another client %q uses the same UID", p.Name)
+	}
+
 	for _, id := range c.ClientIDs {
-		existing, ok := ci.clientIDToUID[id]
+		var existing UID
+		existing, ok = ci.clientIDToUID[id]
 		if ok && existing != c.UID {
-			p := ci.uidToClient[existing]
+			p = ci.uidToClient[existing]
 
 			return fmt.Errorf("another client %q uses the same ID %q", p.Name, id)
 		}

internal/home/clients.go

@@ -286,7 +286,7 @@ func (clients *clientsContainer) addFromConfig(
 
 		_, err = clients.add(cli)
 		if err != nil {
-			log.Error("clients: adding client at index %d %s: %s", i, cli.Name, err)
+			return fmt.Errorf("clients: adding client at index %d %s: %w", i, cli.Name, err)
 		}
 	}