graduate admissions - Is it ok for one of your letters of recommendation to come from someone in the department/program to which you are applying?

[See title.] It seems ok, but one might consider it unfair in some sense? I just wanted to make sure this is acceptable. (I am specifically talking about graduate school applications.)

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Edit: I apologize for previously withholding some information. Here are some more details.

• I am majoring in math and will have two letters from math professors at my undergrad institution.


• I also have a letter from a professor in a different department ("probability/statistics") in the same institution and am applying to this department's graduate program.