Lambda with image running multiple times - python-3.x

Question

I have created a docker image to run some python code on a Lambda that essentially looks like this
FROM public.ecr.aws/lambda/python as build
COPY . .
RUN pip3 install -r requirements.txt
RUN chmod +x run.sh
ENTRYPOINT [ "./run.sh" ]
And it's calling run.sh which looks like this:
#!/bin/bash
# Pull the code from S3
aws s3 cp s3://$S3_REPO/ /tmp/ --recursive --exclude "*" --include "*.py"
cd /tmp
# Run the main.py file Run the main_handler function
python3 -c "from main import run; run()"
Which in turn is calling a python function that looks something like this:
def run():
BUCKET_NAME = "this-bucket"
file_name = f"{datetime.datetime.utcnow()}.json"
output = somefunction()
# Create an S3 client
s3 = boto3.client('s3')
# Convert the output dictionary to a JSON string
output_json = json.dumps(output)
# Check if the bucket exists
try:
s3.head_bucket(Bucket=BUCKET_NAME)
except s3.exceptions.NoSuchBucket:
# Create the bucket if it does not exist
s3.create_bucket(Bucket=BUCKET_NAME)
# Upload the object to the bucket
s3.put_object(Bucket=BUCKET_NAME, Key=file_name, Body=output_json)
return 0
Now that all seemed pretty straight forward, but I have two problems I have been wrestling with (and trying to get assistance with via ChatGPT).
When the lambda runs it always gives this error:
{
"errorType": "Runtime.ExitError",
"errorMessage": "RequestId: c964c24b-db15-4b37-87fd-27406041f85a Error: Runtime exited without providing a reason"
}
But! the code has executed correctly and my expected data is in the S3 bucket.... but twice, about 10 - 15 seconds apart.
The logs indicate that the python function has been run twice! every time. I can confirm that asynchronous invocation has been set to 0 retries. I have been struggling with this for a few days now, I'm very keen to be pointed in the right direction as I am wildly confused and not keen on doubling my s3 usage unnecessarily.
EDIT
Adding this to the bash script confirms that Python is running and exiting successfully. It appears to be an issue with exiting the bash script itself?
if python3 -c "from main import run; run()"
then echo "Success!"
fi