After thinking a while on this, I realized this is not only about merged questions from same OP, but also about duplicates from same OP. The merged ones are more appealing though, because they are locked posts and hence, not qualified for auto-deletion.
Most times (there are exceptions) this will happen because OP asks a question, then nobody answers it (or answer in a way OP is not satisfied), then OP asks the same question again instead of editing/improving the original one. Those are not useful to have around because as written by the same OP, stub and master are likely to have the same wording, hence, not useful for broadening the search scope (and consequently finding the canonical answer).
With that in mind I wrote a query in Stack Exchange Data Explorer (SEDE*) to find duplicate unanswered questions asked by the same OP which outputs the list of stub posts. The unanswered aspect is because we can evaluate if it is a useless post (equal to the master) and flag it for moderator attention and ask it to be purged without being worried with a valid answer being deleted.
Now (jan/2019), the query is outputting 200 candidate questions for deletion:
Duplicate unanswered questions with same OP in source and destination (list of source Qs)
[This is a work in progress, after the clean is done, I plan to find dupes from same OPs, but with answers in the stub, so they can be merged and purged as well]
*SEDE is updated in a weekly basis.