Related to this thread--https://www.efupw.com/forums/index.php?topic=699046.0
Not actually sure this is a bug myself because I haven't noticed any stat issues. PC is a cleric using the damaged wrap of wisdom (-1 con, +1 wis) who then casts owl's wisdom. Upon casting the spell , a message will pop-up saying something to the effect of 'Identical abilities--effects will not stack!' They do seem to stack, however, and I haven't noticed it affecting any other attributes, but admittedly, I haven't been paying much attention.
I've only seen this message come up recently, and I have not seen the pop-up appear on subsequent casts. If I notice anything weird I'll update.
Based on the first post and for Paha's knowledge - This is an old system that used to prevent multiple stat bonuses from stacking. I BELIEVE the intention was to prevent people from using two +1 items together to get +2, but it sadly impacted spells. It seems to pop up rather randomly and usually doesn't take effect, but sometimes it appears to effect players in an odd way (reducing a stat roll by 1 when using a bull strength for example).
Further, I think you can continue to lose 1 strength until you have no bonus by unequipping said item and equipping it again, similar to bonus spell slot items.
Overall it probably doesn't even need to be in place. Most anyone receiving +2 to a stat would be getting it from a DM anyway who can judge the power of said item as they hand it out.