If an index is a duplicate of another index then ...
Visit If an index is a duplicate of another index then Access/Jet does not take any extra space to read the article and download a zip file containing the sample MDB to see for yourself.
I do have a tool available to find duplicate indexes but it needs a few hours of polishing. This is particularly useful if you hit the maximum 32 indexes per Access table and get the following message. "The operation failed. There are too many indexes on table '<table name>'. Delete some of the index on the table and try the operation again." If you are interested in the tool email me and I'll do the polishing.