I know this is an old post but I had this happen after upgrading Visual Studio 2022.

It was giving the ambiguous reference for System.Net.Http. (This relates a little to another post around negut package.)

Insight to project: REST API is being build using .net framework 4.6.2. (important to issue)

Visual Studio 2022 updated and added .net framework 4.6.1, which made that component being found 2 locations that both had that System.Net.Http.

I ventured in the location where the two references were and was able to link the date to that addtion/VS Update. (Right click on the area with ambiguous reference and it shows you were they are)

So the fix in my scenario was to delete the newly added 4.6.1 framework that was added from the VS update.

This may/may not come up for newer versions of visual studio updates.

Hope this is useful for someone.