M: Script is not idempotent - #5

Description

Running challenge-m.sh multiple times currently causes the script to append duplicate content to the output file.

This makes the script unsafe to run more than once and unsuitable for automation.

The script must be made idempotent, meaning repeated executions produce the same result without duplication.

Acceptance Criteria

• Running the script multiple times produces the same output every time
• No duplicate entries are written to the output file
• Existing valid data is preserved and not corrupted
• The script behaves correctly whether the output file already exists or not

Notes

• Assume this script may be executed automatically (e.g. via cron)
• The solution should be simple and easy to reason about
• Idempotency is more important than performance