Fix double membership=join event handling

This fixes the problem mentioned in 8e1aa5b11c.

Turns out that it's a long-standing Synapse bug:
https://github.com/matrix-org/synapse/issues/9768

bot/bot.go

@@ -19,14 +19,15 @@ import (
 
 // Bot represents matrix bot
 type Bot struct {
-	noowner    bool
-	federation bool
-	prefix     string
-	domain     string
-	rooms      map[string]id.RoomID
-	roomsmu    *sync.Mutex
-	log        *logger.Logger
-	lp         *linkpearl.Linkpearl
+	noowner       bool
+	federation    bool
+	prefix        string
+	domain        string
+	rooms         map[string]id.RoomID
+	roomsmu       *sync.Mutex
+	log           *logger.Logger
+	lp            *linkpearl.Linkpearl
+	handledEvents sync.Map
 }
 
 // New creates a new matrix bot