Unexpected HTTP response 409 error

I am getting this error when adding a trigger tag to a contact to enter them into a campaign, even though I know the contact did not already have the tag:
Unexpected HTTP response code: 409 (Conflict)
Response body: {“category”:“Client Error”,“developerMessage”:“Tag could not be applied. Most likely tag already applied to contact.”,“userMessage”:“Contact tag not applied.”,“apiMessageNumber”:41907}

its for this action:
Date Invite Tag Applied

Any help would be much appreciated!

Thanks, Liz