servo: Merge #19325 - Report an errror if a package has duplicates allowed but there are no duplicates (from m-novikov:tidy-ignore-without-duplicates); r=jdm

Resolves: #19306

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #19306 (github issue number if applicable).

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- [X] There are tests for these changes

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Source-Repo: https://github.com/servo/servo
Source-Revision: a812af51d614bd96402da91d8f648ce51b301a1b

servo/python/tidy/servo_tidy/tidy.py

@@ -339,10 +339,17 @@ def check_lock(file_name, contents):
         packages_by_name.setdefault(package["name"], []).append((package["version"], source))
 
     for (name, packages) in packages_by_name.iteritems():
-        if name in exceptions or len(packages) <= 1:
+        has_duplicates = len(packages) > 1
+        duplicates_allowed = name in exceptions
+
+        if has_duplicates == duplicates_allowed:
             continue
 
-        message = "duplicate versions for package `{}`".format(name)
+        if duplicates_allowed:
+            message = 'duplicates for `{}` are allowed, but only single version found'.format(name)
+        else:
+            message = "duplicate versions for package `{}`".format(name)
+
         packages.sort()
         packages_dependencies = list(find_reverse_dependencies(name, content))
         for version, source in packages: