nip61: perhaps simplify the function signature and support specific source mints.

utils.go

@@ -5,6 +5,7 @@ import (
 	"cmp"
 	"encoding/hex"
 	"net/url"
+	"slices"
 )
 
 // IsValidRelayURL checks if a URL is a valid relay URL (ws:// or wss://).
@@ -46,3 +47,15 @@ func CompareEventReverse(b, a Event) int {
 	}
 	return cmp.Compare(a.CreatedAt, b.CreatedAt)
 }
+
+// AppendUnique adds items to an array only if they don't already exist in the array.
+// Returns the modified array.
+func AppendUnique[I comparable](arr []I, item ...I) []I {
+	for _, item := range item {
+		if slices.Contains(arr, item) {
+			return arr
+		}
+		arr = append(arr, item)
+	}
+	return arr
+}