Field validators are called multiple times

[jps-ob]

In the below example (ignoring programming logic) field validators are called multiple times. Is this bug or not?

In [201]: def is_names_empty(value):
     ...:     print("Is Names Empty?")
     ...:     return value

In [202]: def is_duplicate(value):
     ...:     print("Is Duplicate?")
     ...:     return value

In [203]: class A(Model):
     ...:     id=StringType()
     ...:     names=ListType(StringType, validators=[is_names_empty])
     ...:     black_list=ListType(StringType, validators=[is_duplicate])
     ...:
     ...:     def validate_names(self, validated, new_data):
     ...:         print(f"New Data: {new_data}")
     ...:         print(f"Validated: {validated}")
     ...:         if any([n in validated["black_list"] for n in new_data]):
     ...:             raise ValidationError("Blacklisted name found")

In [204]: a=A();a.id=1;a.names=["kk"];a.black_list=["jj"];a.validate()
Is Names Empty?
Is Names Empty?
Is Duplicate?
Is Duplicate?
New Data: ['kk']
Validated: {'id': '1', 'names': ['kk'], 'black_list': ['jj']}

In [205]: a=A();a.id=1;a.names=["kk"];a.black_list=["jj","kk"];a.validate()
Is Names Empty?
Is Names Empty?
Is Duplicate?
Is Duplicate?
Is Duplicate?
New Data: ['kk']
Validated: {'id': '1', 'names': ['kk'], 'black_list': ['jj', 'kk']}
---------------------------------------------------------------------------
DataError                                 Traceback (most recent call last)
<< Truncated trackeback>>
DataError: {"names": ["Blacklisted name found"]}

Even if model validator has to call field validator, why is_duplicate is called 3 times ?