Observed behaviour: after obtaining the Geas Removal Scroll and giving it to Lothander to obtain his half of the antidote and the information about where to find Marek, I quickly killed Lothander (there's no rep loss) before he left (mainly in order to loot the second pair of Boots of Speed). I then confronted Marek and obtained the rest of the antidote, thus being cured and completing the Marek & Lothander quest. All working fine so far ... BUT considerably later, I set off on Alatos's quest to loot Oberan's estate, so I went to Central BG and was walking through the gate/tunnel in the city's inner wall when I happened to pause for a moment ... which brought up a blue circle for a character standing in the tunnel but out of sight because he was concealed by the stonework of the tunnel. It was Lothander! Speaking to him he said "What were you doing?! You asked the wrong question! You idiot, now we're all dead." (I'm not sure what he was referring to, but maybe that's one of his dialogues if you don't complete his quest correctly? Even though actually I had completed it.) He offered no conversation option, so I tried attacking him ... and ended up killing him again, and not only got the XP again but also his corpse dropped Boots of Speed (and his other equipment) again! Now, with the original pair obtained from Drasus, I've got three pairs of Boots of Speed! That surely can't be intended. (Actually, I'm not even sure whether it was intended that I could obtain the second pair without failing the Marek & Lothander quest and thereby dying).
Expected behaviour: if Lothander is killed, he should drop his equipment and then stay dead, not reappearing (and especially not reappearing with duplicates of his original equipment).