I just completely rewrote the "Add a plant" page in the database. The duplicate checking is now MUCH stronger, and the whole experience should be much friendlier.
Additionally, starting now the duplicate checker is active for everyone. If you previously had a special case where the duplicate checker was disabled for you, that is no longer the case. What is new now is that if there is a duplicate match on the plant you're trying to add, it will give you a warning, telling you about the duplicate, but then it lets you go ahead and submit the new plant even though the potential duplicate does exist.
This option is available to everyone. So, moderators: keep an eye on the proposals. If they bypassed the duplicate checker, it will say so in the moderation queue under a heading that says, "Possible dup".
This is part 1 of my reworking of this part of the database. Part 2 will be where I update the "Edit plant name" area.