can't copy a key to the same key

cmd_generic.go

@@ -604,6 +604,11 @@ func (m *Miniredis) cmdCopy(c *server.Peer, cmd string, args []string) {
 			toDB = fromDB
 		}
 
+		if fromDB == toDB && opts.from == opts.to {
+			c.WriteError("ERR source and destination objects are the same")
+			return
+		}
+
 		if !m.db(fromDB).exists(opts.from) {
 			c.WriteInt(0)
 			return

integration/generic_test.go

@@ -337,6 +337,20 @@ func TestCopy(t *testing.T) {
 			c.Do("TYPE", "replaceme")
 			c.Do("GET", "replaceme")
 		})
+		c.Do("SELECT", "0")
+
+		t.Run("copy to self", func(t *testing.T) {
+			// copy to self is never allowed
+			c.Do("SET", "double", "1")
+			c.Error("the same", "COPY", "double", "double")
+			c.Error("the same", "COPY", "double", "double", "REPLACE")
+			c.Do("COPY", "double", "double", "DB", "2") // different DB is fine
+			c.Do("SELECT", "2")
+			c.Do("TYPE", "double")
+
+			c.Error("the same", "COPY", "noexisting", "noexisting") // "copy to self?" check comes before key check
+		})
+		c.Do("SELECT", "0")
 
 		// deep copies?
 		t.Run("hash", func(t *testing.T) {