What that error means is that you bought something with iTunes, that purchase was deposited to a certain Eventide user account like for example "testUser1". If you switch to a different eventide account like "testUser2" and restore your purchases, that's going to cause problems, because that purchase was already deposited in the account of "testUser1". If you try to buy the same algorithm again after switching Eventide accounts, I'm not 100% sure what that will do, but it will likely just try to restore your previous purchase, and you'll end up in this same situation.
You mentioned earlier on the forums that you're going to want to have more than five H9s, and so, you're planning on splitting them between multiple Eventide accounts. You're going to run into problems purchasing algorithms through iTunes with this set-up. You should use the desktop version of H9 Control for your purchases instead if this is what you're trying to do, because Amazon's payments system works differently than Apple's iTunes. And you won't run into these problems.