Does SwiftKey replace double spaces with a period/full-stop?

It’s up to you.

If “Spacebar completion mode” in Advanced settings is set to “always insert a space” or “complete current word” then you have the option to enable Quick ‘.’, which means that double-tapping the spacebar will insert a period followed by a space.

If spacebar is set to “always insert a prediction” then repeatedly hitting the spacebar will continue to insert the middle prediction.

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