If a friend sends me a text message saying “message 1” and 10 seconds later he sends me another text message saying “message 2” the following happens.
The first message appears on the Pebble screen. 10 seconds later the second message appears. If while viewing the second message I press Select and then Dismiss the second message disappears and the first message reappears. If while viewing the first message I press Select and Dismiss the I get the message Failed and the first message remains on the watch.
This happens with standard text messages and messages from Signal.
If I get two quick messages from different senders, this does not happen.