UA has an overweight fee, as well an oversize fee. These fees are mutually exclusive - so in a situation like yours, they will charge both. The UA site lists the charges for an oversize bag and an overweight bag, and nowhere does it say they will somehow combine them, or charge the lowest/highest of the charges - they just charge them both. Don't know what you expected. WNs site does have a footnote on their fees page that notes there is only one $75 fee applied if the bag is overweight and oversize.

I know DL normally gives a 1 pound exception. Don't know that UA does - therefore, they seem to have charged what they should have. The UA page does appear to have a typo somewhere - though not sure where - they say 51.0 to 70.0 pounds is $100 extra, and 71.0 to 100.0 is $200 extra. One of those groups should apply to 70.1 to 70.9 pounds, not sure which.

Either way, your post title and post itself are not accurate - you were not 'double' charged by UA - you were charged single oversize and single overweight fees, per UA policy.
It's also not 'hidden' - these charges are laid out on UAs site. While you're comparing...looks like UA allows bags up to 100lbs, while WN only allows 80.

Different airlines have different policies on baggage (and other things). If the bag fees for the luggage you were traveling with were better with WN then UA, then you should have considered that when deciding who to book with. Certainly fair not to like the amount UA is charging - but accusing them of 'double' charging and 'hidden' charges is just not true.