Or HaChaim on Numbers 21:2/אור החיים על במדבר כ״א:ב׳

1 אם נתן תתן, "If You will surely deliver, etc." The reason the Israelites repeated the words נתן תתן, is due to the fact that they were not certain which nation they were talking about. They said "if" i.e. if this nation is one of the seven Canaanite nations, all well and good. If not, Israel asked that G'd should nevertheless תתן, give that people into its hands, etc. There is a comment in the Midrash Lekach Tov according to which the first נתן refers to G'd handing back the prisoner the Amalekites had captured.