The part that matters is whether his closing was on a property you ever touched. If it was something you sourced and he bought it through a different wholesaler, that's a list problem. If it was a property you never had under contract and he found it another way, that's just two pipelines running at the same time and you can't really hold that against him.
I had a buyer in Cleveland same situation, Slavic Village, SFRs in the 900-1100 a month rent range, three passes from me in about four months. He closed on something in that zip through a direct mail seller before I ever got to it. Stung at the time but when I pulled the deed he paid 12k more than what I would have had it under contract for, so my sourcing wasn't slow, his tolerance for retail just went up when he got impatient. That's actually useful information. Means next time I know he'll stretch on price when he's been sitting too long, so I started bringing him deals with a little less margin cushion and closed two back to back. The duplicate list question is almost beside the point if you don't know his actual ceiling when he's been dry for 90 days.