[PW] ?onion

[Reference]

Hello Wombats!

What are words which have the same pair of letters at the beginning and the end called? Example : onion.

This question was posed to the Florida Ask-a-Librarian service. I have not seen an answer posted yet.  I have been looking in the Oxford companion to English Usage under word (The word theme). I have looked up terms that are not familiar to me, but it is a long list, and I am hoping there is a Wombat out there who knows "off the top of their head".

TIA