Added validation check for existing submission.

src/submission/submit.py

@@ -147,6 +147,12 @@ def submit_model(
         return styled_error(
             f"Daily submission limit of {DAILY_SUBMISSION_LIMIT_PER_USER} reached. Please try again in \n {time_str}."
         )
+
+    if f"{username}/{model_name}" in get_user_submission_names(competition_type, profile):
+        return styled_error(
+            f"Submission {model_name} already exists. Please use a different name for your submission."
+        )
+
     try:
         submission = create_submission(
             username=username,
@@ -183,7 +189,7 @@ def submit_model(
         return styled_error(f"Error submitting model: {str(e)}")
 
 
-def load_demo_example(model_name: str, competition_type: CompetitionType) -> Workflow:
+def load_demo_example(model_name: str, competition_type: CompetitionType) -> Workflow | TossupWorkflow:
     """Load a demo example submission."""
     examples_dir = f"{EXAMPLES_PATH}/{competition_type}"
     filepath = f"{examples_dir}/{model_name}.yaml"