Do I have a right to say be mindful of my time?

I've been covering for our two receptionists when they sometimes have breaks together.

They always. take advantage of me when my boss says to be mindful of my time.

They've always at least 10 to 15 mins over and come back with a fake apology.

My boss is on leave and I the receptionists have been taking advantage of me all week cause I can't say no.

I was thinking to cc my acting boss as she doesn't even know they're going out for lunch.

They need to tell someone that someone else covering for them if they're both not there which they haven't.

Can I say:

So and so are taking lunch together while I'm doing both their cover. If they could be mindful of my time and stick within the required 1 hour lunch break. The last few times recent both have been 10 minutes overtime.

I don't mean to be a snitch but they've taken advantage of me all the time and I'm just tired of it.

Do I have a right to say be mindful of my time?
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