@kit is correct, to expand on his answer ...
I see the issue is caused by having a 1.6.0-develop.1 after 1.6.0 was released. The next version after the 1.6.0 release must be one of:
NuGet enforces that you can only ever publish a 1.6.0 package once. So in this case "release" is when you publish to NuGet :-). You can build as many 1.6.0 as you like but you can only release one of those once.
The important question is what does your team define as a release? (such a big question)