It appears that on Motorola Droid (or Android 2 in general?) Gmail pulls and pairs messages into conversation based on a subject even if they are in Trash.

When you receive two shopping list emails in a row that are very similar then Gmail will "quote" the lines that appear in both messages. This seems to cause the problem.

Normally, you shouldn't be getting two very similar shopping list messages anyway. It would duplicate your items.

This is currently under investigation.