added form level checking for duplicate entries in nameserver form

src/registrar/forms/domain.py

@@ -117,8 +117,34 @@ class DomainNameserverForm(forms.Form):
                 self.add_error("ip", str(e))
 
 
+class BaseNameserverFormset(forms.BaseFormSet):
+    def clean(self):
+        """
+        Check for duplicate entries in the formset.
+        """
+        if any(self.errors):
+            # Don't bother validating the formset unless each form is valid on its own
+            return
+
+        data = []
+        duplicates = []
+
+        for form in self.forms:
+            if form.cleaned_data:
+                value = form.cleaned_data['server']
+                if value in data:
+                    form.add_error(
+                        "server",
+                        NameserverError(code=nsErrorCodes.DUPLICATE_HOST, nameserver=value),
+                    )
+                    duplicates.append(value)
+                else:
+                    data.append(value)
+
+
 NameserverFormset = formset_factory(
     DomainNameserverForm,
+    formset=BaseNameserverFormset,
     extra=1,
     max_num=13,
     validate_max=True,

src/registrar/utility/errors.py

@@ -68,7 +68,8 @@ class NameserverErrorCodes(IntEnum):
         - 4 TOO_MANY_HOSTS  more than the max allowed host values
         - 5 MISSING_HOST host is missing for a nameserver
         - 6 INVALID_HOST host is invalid for a nameserver
-        - 7 BAD_DATA bad data input for nameserver
+        - 7 DUPLICATE_HOST host is a duplicate
+        - 8 BAD_DATA bad data input for nameserver
     """
 
     MISSING_IP = 1
@@ -77,7 +78,8 @@ class NameserverErrorCodes(IntEnum):
     TOO_MANY_HOSTS = 4
     MISSING_HOST = 5
     INVALID_HOST = 6
-    BAD_DATA = 7
+    DUPLICATE_HOST = 7
+    BAD_DATA = 8
 
 
 class NameserverError(Exception):
@@ -93,6 +95,7 @@ class NameserverError(Exception):
         NameserverErrorCodes.TOO_MANY_HOSTS: ("Too many hosts provided, you may not have more than 13 nameservers."),
         NameserverErrorCodes.MISSING_HOST: ("Name server must be provided to enter IP address."),
         NameserverErrorCodes.INVALID_HOST: ("Enter a name server in the required format, like ns1.example.com"),
+        NameserverErrorCodes.DUPLICATE_HOST: ("Remove duplicate entry"),
         NameserverErrorCodes.BAD_DATA: (
             "There’s something wrong with the name server information you provided. "
             "If you need help email us at help@get.gov."