1

"If you have any doubt", let me know." "If you have any doubts, let me know."

Both sound correct. Are there any subtle differences in meaning?

closed as off-topic by Edwin Ashworth, FumbleFingers, tchrist, Centaurus, Chenmunka Mar 21 '15 at 18:31

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2

Doubt can be both a mass noun and a countable noun.

So one can be filled with doubt (mass noun), one can have a a number of doubts about something (count noun), and one can also have just a single doubt about the whole complex process (count noun).

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