Well, to copy my comment from Discord:
Okay, I think I found another minor bug, though I'm willing to be proved wrong.
The following is from the text of a circlet of persuasion: grants its wearer a +3 competence bonus on Charisma-based checks
And the following is from the ring of eloquence: granting a +2 competence bonus on Bluff, Diplomacy, Intimidate, and Perform (oratory) checks.
On the troubleshooting sheet for my Bard that is showing a +13 Bluff, I have the following: Bluff +13 = +1[RANK] +1[Gift of Tongues] +2[TYPE=COMPETENCE|Ring of Eloquence] +3[cskill BLUFF GTEQ 1,skill BLUFF GTEQ 1|TYPE=CLASSSKILL|Bluff] +3[STAT] +3[TYPE.Charisma]
As you can see, both grant competence bonuses so probably should not stack. Though, as I said, I'm definitely willing to be proved wrong.