Pregnancy does not prevent impregnation until passage updates again
Certain slave interactions don't seem to update the appropriate values to prevent impregnation until the interact passage has been refreshed again by other means, similar to #5649 (closed), though that's been an issue for quite a long time, while this appears to be new. I would assume something to do with humungusluver's recent work on fertility, but I'm not sure.
To give a specific example, remaining on the interact page, take a non-pregnant fertile slave. Click "Impregnate ${him} yourself" for fPCImpreg, check her pregnancy status in console (let's say she's now pregnant with triplets), and notice that fPCImpreg is still on the list of available interactions; click it again, check her pregnancy again and see that she's now carrying octuplets, and note that fPCImpreg has now disappeared from the list. Congratulations, you've double-impregnated your slave.
This occurs regardless of genetic quirks or modifications. Slaves with superfetation can have their pregnancy added to twice in the same week, just like normal slaves can be impregnated twice.
Impregnating the slave and then, say, kissing her, also removes impregnation actions from the list. Taking some other actions that can cause pregnancy, such as fVagina, fButt, and (with mpreg) fAnus, cannot cause double pregnancy themselves due to using tryKnockMeUp instead of knockMeUp, but do still keep fPCImpreg available (and able to reimpregnate) until one extra update has occurred. fSlaveSlaveImpreg is an interesting case, correctly making other impregnation options unavailable on the first go, but aborting and lying to you that you have no virile slaves if you try to use it after impregnating the slave by other means. fSlaveSelfImpreg behaves exactly like fPCImpreg.
All testing was done with V.menstruation disabled. Oddly, when I enabled it, fPCImpreg, fSlaveSlaveImpreg, and fSlaveSelfImpreg all totally disappeared from my fertile slaves' interaction options, even though they could still get knocked up through fVagina and such; because of this, I was unable to test if double-impregnation was possible under that setting.