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Can I say,

After a / an one-hour break in the afternoon, we all went home.
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Comments  (Page 2) 
I will be home in an one hour or I will be home in a one hour?
Not correct.
Say
eg I will be home in an hour.
eg I will be home in one hour.
I will be home in an Hour. "An" already indicates the quantity of one.

I guess that in the other cases of "a one-hour break" the "one-hour" is working like an adjective that modifies "break" and the article "a" is for the "break" in itself, making it (one) break. Hope that makes sense.
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Such a one as this
BarbaraPA(Why wouldn't you go home at the start of the break? We stopped work for an hour. Then, when that hour was over, even though we were still at work, we went home. If you are going to leave immediately after the break, just leave when you start the break.)

If you have to punch out, you're going to have to wait for that hour before leaving.