fix: prevent dpulicate entries in owners files for global owners

this patch refactors the _GLOBAL_ODEOWNERS variable to a list,
additionally refactoring the logic when appending _GLOBAL_OWNERS to the
`teams` property of a `codeowners()` rule to ensure that users in
_GLOBAL_OWNERS are not duplicated in the generated output.

tools/defaults.bzl

@@ -44,7 +44,9 @@ def npm_package(**kwargs):
         **kwargs
     )
 
-_GLOBAL_OWNERS = "@alexeagle"
+_GLOBAL_OWNERS = [
+    "@alexeagle",
+]
 
 def codeowners(name = "OWNERS", no_parent = False, **kwargs):
     """Convenience macro to set some defaults
@@ -62,7 +64,7 @@ def codeowners(name = "OWNERS", no_parent = False, **kwargs):
 
     # Googlers: see http://go/owners#noparent
     if not no_parent:
-        teams.append(_GLOBAL_OWNERS)
+        teams += [owner for owner in _GLOBAL_OWNERS if owner not in teams]
 
     _codeowners(
         name = name,