Agents don’t delete anything, so consequently we have a record of every query we’ve received. In my excavation of various folders, I discovered some writers have queried the same book to the same agent twice.
Some months passed, the title changed, and there were probably changes in the manuscript. But nowhere in the query did it say they had queried this agent before or received a rejection.
You may be tempted to leave off that information. Will the agent actually remember you? Won’t they reject without reading if they see they’ve rejected you before?
No, the agent may not remember you, but they will know they’ve seen a story like this before. It will tickle their brain and get them thinking this isn’t an original concept–which is not what you want them to think.
However, if you write a brief sentence telling the truth* the agent may decide to read through again in the hope that you solved whatever problems put them off the first time. They may not read it, though, and that’s probably because it’s just not the right genre or premise for them. In which case, there’s not much you could’ve done.
Agents only really care that your book is well-written, engaging, and original. If you managed to make it that since last time they saw it, they’ll be happy to look again.
* If you claim you edited heavily, you better have edited heavily. Please don’t lie. You look so much better being honest.