Are those emails actually downloading that number of times or are old deleted emails being resurrected? You may be emptying Junk, but do you compact the Junk folder after emptying it, to remove all traces of deleted junk. If no, then right click on Junk folder and select 'compact'.

Are you talking about a pop or imap mail account? How often do you compact folders?

Thank you. Answer to question 1 - no idea. 2 - I do compact now and then when Thunderbird pushes me 3 - pop account I did some searching online and found someone with similar problem with their inbox. The solution, which applied here it seems, was to right click on the folder (junk in this case) select properties, and lo, there in properties is a button to "repair"! One click, didn't seem like anything happened, but now the junk is coming in singly and not 20 at a time. So I believe you are right as was the other solution. THANK YOU!

Unfortunately that solution appears very temporary. I have been compacting the junk and all folders regularly, and repairing the junk folder frequently. It might work for a day, and then I'll again get the multiple copies. 375 emails this morning from about a dozen sources. All copies of a particular source appear identical. Viewing them online or on my phone reveal only one copy of each. Any more suggestions, anyone?