All formatters in a compilation are automatically added to a source generated resolver so that it can be found at runtime.

When two formatters implement IMessagePackFormatter<T> for the same T, it cannot be statically determined which formatter should be used, and this diagnostic results.

Either remove all but one of the conflicting formatters, or exclude all but one from inclusion in the source generated resolver by applying the [ExcludeFormatterFromSourceGeneratedResolver] attribute to some of the formatters.