Fix duplicate record assignment

[apauly]

Hi,

I found the following issue when using jit_preloader:

If multiple records in a collection have the same record set for a belongs_to, which itself has a has_many association, the has_many association will end up having duplicate records assigned. When using ARs preload, there are no duplicate records.

I could reproduce this behaviour with a spec and was also able to fix it. However, I'm not 100% sure if my fix will add other side effects if there are actually duplicate records via has_many through: or similar setups.

Maybe somebody else with better understanding of the gem itself can simply tell where to apply a fix without side effects.

[fimmtiu]

@apauly Thank you for the bug report and the pull request! We quite appreciate the feedback.

I can duplicate the issue under Rails 6 now. (Seems to be okay on Rails 5.) I'm not sure that this is the best approach for fixing it, however, as it still makes twice as many queries as it needs to, then throws away half the results. (By the time we get to the uniq call, we've already made a duplicate phone_numbers query.)

I'll open an issue for it (#32) and we'll try to get it to it soonish.