Compiler Error CS0409
A constraint clause has already been specified for type parameter 'type parameter'. All of the constraints for a type parameter must be specified in a single where clause.
Multiple constraint clauses (where clauses) were found for a single type parameter. Remove the extraneous where clause, or correct the where clauses so that a unique type parameter in each clause.
// CS0409.cs
interface I
{
}
class C<T1, T2> where T1 : I where T1 : I // CS0409 – T1 used twice
{
}