I believe there was a period in the 1980s when returns were sent to GRO by photocopying the local entries. How GRO managed them in their system I have no idea. Each entry would be an A4 copy so the old system of having a number (10) of entries per page would no longer apply, but GRO carried on using page numbers, at least for a while.
It does mean that for this period you may actually get a copy of the original record when ordering a certificate from GRO - something that doesn't apply at any other time.
The duplicates in the indexes are usually explained by names being recorded on register entries as "X otherwise (or formerly known as) Y", something registrars are encouraged to do. The entry then appears in the index under each name variation.
My own mother's death, in 2022, appears 3 times in the GRO index for that reason.