Here's a bash script that implements the described system using the AWS CLI:

```
#!/bin/bash

# Define the S3 bucket and key of the uploaded file
bucket_name="your-s3-bucket"
object_key="path/to/your/uploaded/file.jpg"

# Check if the resized object already exists in the S3 bucket
resized_object_key="${object_key%.*}-resized.${object_key##*.}"
aws s3 ls "s3://$bucket_name/$resized_object_key" &> /dev/null
if [ $? -ne 0 ]; then
  # Resize the uploaded object and store the resized object in the S3 bucket
  # using the same key as the uploaded object but with a different file extension
  aws s3 cp "s3://$bucket_name/$object_key" - | \
    convert - -resize 800x800\> "s3://$bucket_name/$resized_object_key"
fi
```

This script assumes that you have the AWS CLI and the ImageMagick convert tool installed on your machine. You'll need to replace your-s3-bucket and path/to/your/uploaded/file.jpg with the actual names of your S3 bucket and uploaded file, respectively.

This script checks if the resized object already exists in the S3 bucket by using the aws s3 ls command and checking its exit code. If the resized object does not exist, the script uses the aws s3 cp command to copy the uploaded object from the S3 bucket to the standard output, pipes it through the ImageMagick convert tool to resize the image, and finally uses the aws s3 cp command to store the resized object in the S3 bucket using the same key as the uploaded object but with a different file extension.
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