This bug also occured to me when I tested my /sleep and /xrun, that use a COM object to delay mIRC, before submitting them to ms.org. This is the note I eventually added, to inform users of this behavior:
  • When calling /xrun from an event, expect error such as the event being called twice. One way around this is creating an alias that contains your commands and calling it from the event via /timer 1 0 <alias>