Is "mention in passing" redundant?

The definitions for mention Macmillan lists first are certainly commonly used ones:

mention [verb] [transitive]

(1) to say something during a conversation, but not discuss it much or give many details

• He didn’t mention her all evening.

(1b) to refer to something in a written document without giving many details ...

mentioned above/below (=mentioned earlier or later in a document)

The 'not discussing it much' condition probably mandates the 'only in passing' descriptor in conversation (though it could be argued that it was one person's sole short contribution and the descriptor is unwarranted), but the document example demands that it be 'in passing'.

So it can certainly be argued that 'in passing' repeats the 'not discussing much / not giving many details' assertion.

However, redundancy is certainly not unacceptable per se, and may be the preferred form. "I'll mention that ..." can sound unnatural as an aside.

"I'll just mention that ..." is often used to introduce an aside.

And "I'll just mention in passing that ..." usually sounds more natural than "I'll just mention that".

The phrase is not redundant.

If you mention something in passing, it means that you insert the mention into a train of thought or argument, rather then merely mentioning it out of any context. Something merely "mentioned" has no context, whereas the phrase "mentioned in passing" is most useful when placed in a surrounding context.

I must mention in passing that it would have helped us answer your question if you had given an example of the use of the phrase.