vatin: reject a duplicated country code prefix (#420)

stdnum/vatin.py

@@ -87,11 +87,26 @@ def validate(number: str) -> str:
     This performs the country-specific check for the number.
     """
     number = clean(number, '').strip()
-    module = _get_cc_module(number[:2])
+    cc = number[:2]
+    module = _get_cc_module(cc)
     try:
-        return number[:2].upper() + module.validate(number[2:])
+        # Most country modules accept and strip the optional country-code
+        # prefix themselves, so the full number is validated. Stripping the
+        # prefix here as well would silently accept a doubled country code
+        # such as "BE BE 0308.357.159" (see #420).
+        result = module.validate(number)
     except ValidationError:
-        return module.validate(number)
+        # Some country modules expect the national number without the country
+        # code prefix. Only retry that way when the remainder is not itself
+        # prefixed with the country code, otherwise a doubled prefix would be
+        # accepted.
+        remainder = re.sub(r'[^0-9A-Za-z]', '', number[2:])
+        if remainder[:2].upper() == cc.upper():
+            raise
+        result = module.validate(number[2:])
+    if not result.startswith(cc.upper()):
+        result = cc.upper() + result
+    return result
 
 
 def is_valid(number: str) -> bool:

tests/test_vatin.doctest

@@ -102,3 +102,19 @@ Check for VAT numbers that cannot be compacted without EU prefix:
 True
 >>> vatin.compact('EU191849184')
 'EU191849184'
+
+
+A duplicated country code prefix should not be accepted (#420). This used to
+pass because the country code was stripped twice (once here and once by the
+country module):
+
+>>> vatin.is_valid('BE 0308.357.159')
+True
+>>> vatin.is_valid('BE BE 0308.357.159')
+False
+>>> vatin.is_valid('BEBE0308357159')
+False
+>>> vatin.validate('BE BE 0308.357.159')
+Traceback (most recent call last):
+    ...
+InvalidFormat: ...