Stop astype from overwriting arrays when copy=True

[perimosocordiae]

Fixes gh-8678 by adding a new kwarg to the private _deduped_data method, which prevents sum_duplicates from modifying the source matrix when it shouldn't.

[pv]

Test failures

[lesteve]

Interesting, the diff looks fine to me: copy=False in by default _dedup_data should not affect the existing code and the rest of the changes looks good. Any suggestions about why the CIs are failing?

[pv]

You take the simplest-looking failure and run it locally with pdb.set_trace() to find out.

[pv]

... since I guess it's not easy to find out just by looking at it, as the changes themselves look innocent.

[lesteve]

OK thanks for the details, I was asking to see if anyone had some insights off the top off their head just in case.

[pv]

The problem is that a "deduped_data" operation is impossible: deduplication alters also index arrays (previously, it was in-place which is OK). This helper function probably should be renamed to something else, since it's misleading now.

[clementkng]

Hi @pv, I would like to try working towards closing this PR. I took a look at the bug report, and was initially thinking of adding a test to duplicate the behavior and after that work on fixing the build issues. From your comments above, are you implying that the fix to this issue is to name the helper function _deduped_data to something else or would we need to explore an alternate fix to the original bug?

[rth]

@clementkng If you are interested in investigating this that would be great! A test to reproduce the initial issue would be useful in any case.

are you implying that the fix to this issue is to name the helper function _deduped_data to something else or would we need to explore an alternate fix to the original bug?

Just changing the name of that function is not going to help I imagine. I'm not sure how to avoid the issue mentioned in #8679 (comment).

[clementkng]

Thanks @rth. I was just trying to guage based off @pv's comments

The problem is that a "deduped_data" operation is impossible:

whether a solution to the bug was possible at all, or no longer possible due to how deduplication now modifies the index arrays.