Remove double errors

Consider the following function.

mTestFunction :: Int
mTestFunction = length [unknown]

The frontend yields the following errors.

Test.curry:2:1-2:32 Error:
    Ambiguous type variable _4
    in type Prelude.Data _4 => Prelude.Int
    inferred for equation
    mTestFunction = length [unknown]
   | 
 2 | mTestFunction = length [unknown]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Test.curry:2:1-2:32 Error:
    Ambiguous type variable _4
    in type Prelude.Data _4 => Prelude.Int
    inferred for function `mTestFunction'
   | 
 2 | mTestFunction = length [unknown]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ERROR occurred during parsing!

As both errors refer to the same issue (even with the same spans), one error should not be raised.