When a Lambda is deleted, it may not remove the corresponding Event notification / Trigger from the associated Bucket.

To see if this is the case, go to S3 > [Bucket of interest] > Properties > Event Notifications. That list should show all events / triggers associated with the Bucket. It will also show the names of associated Lambda functions (on the far right), so you can see if those are still-active, or familiar-but-deleted Lambdas.

Delete any you don't want, and try the creation again.