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This is a phrase I heard from an American TV show when the scenario is involving mulple things combined in a complex situation and this is what one of the characters blurted out at the moment.

According to the substitles which are translated into my native language,

it means to handle one thing at a time.

Now my question is, is this is a common idiom?

Cuz when I googled it there turned out to be no matching results.
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No, it isn't. Was the show you were watching somehow sci-fi or fantasy-related? I wouldn't be surprised to hear Xena say something like that, but I would be if Jack Bauer said it.
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It's a common American way to say to handle one problem at a time or one after the other, not all together.

It is said many times even into Little Pretty Liars TV series

SB