I have noticed that .c files in libc get built with -I/usr/src/lib/libc
being passed twice.

This is because locale/Makefile.inc adds -I${.CURDIR}, supposedly to
reach files in locale/, but first, all .h files there are only used by
.c files in the same directory and included using "", not <>, and
second, this Makefile fragment gets sourced by the top-level libc
Makefile, so ${.CURDIR} points at libc, not libc/locale.

Index: lib/libc/locale/Makefile.inc
===================================================================
RCS file: /OpenBSD/src/lib/libc/locale/Makefile.inc,v
diff -u -p -r1.28 Makefile.inc
--- lib/libc/locale/Makefile.inc	25 Aug 2023 12:45:45 -0000	1.28
+++ lib/libc/locale/Makefile.inc	6 Aug 2025 12:01:39 -0000
@@ -24,4 +24,4 @@ MAN+=	nl_langinfo.3 setlocale.3 newlocal
 	wcstol.3 wcstombs.3 wcsxfrm.3 wctob.3 wctomb.3 \
 	wctype.3 iswctype.3 wctrans.3 towctrans.3 wcwidth.3
 
-CFLAGS+=-I${.CURDIR} -I${LIBCSRCDIR}/citrus
+CFLAGS+=-I${LIBCSRCDIR}/citrus