Hi,
You can enable the Concurrency Control for the trigger of the flow by clicking the 3 dots button > Settings of the trigger. (Value of the limit = 1)

Here is my example to get the names of the selected contacts and append them to the description of the account with the Expression - concat.

Here is the original description of the account.

After I selected the two contacts in sub-grid and run the flow,

the description is shown as the following:

If this helped you, I'd appreciate it if you'd mark this as a Verified Answer, which may in turn help others as well.
Best Regards,
Nya