buildsystem: keep subfolders of sources unique

Makefile.base

@@ -112,7 +112,7 @@ ifneq (,$(SUBDIRS_IN_DIRS))
     $(patsubst $(CURDIR)/%,./%, $(SUBDIRS_IN_DIRS)). \
     Please select a single approach for each subfolder to prevent linking errors.)
 endif
-SUBDIRS := $(filter-out $(BINDIR)/$(MODULE)/, $(dir $(OBJ)))
+SUBDIRS := $(sort $(filter-out $(BINDIR)/$(MODULE)/, $(dir $(OBJ))))
 
 include $(RIOTMAKE)/blob.inc.mk
 include $(RIOTMAKE)/tools/fixdep.inc.mk

examples/subfolders/Makefile

@@ -13,7 +13,7 @@ USEMODULE += my_module # name as defined in module/Makefile
 
 # Add source files in subfolders manually
 SRC += main.c
-SRC += folder/a.c folder/subfolder/b.c
+SRC += folder/a.c folder/subfolder/b.c folder/subfolder/c.c
 
 # Alternative method to add files in subfolders using wildcards
 # SRC += $(wildcard *.c folder/*.c folder/**/*.c)

examples/subfolders/folder/subfolder/c.c

@@ -0,0 +1,14 @@
+/*
+ * Copyright (C) 2025 ML!PA Consulting GmbH
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+#include <stdio.h>
+
+void folder_c(void)
+{
+    puts("./folder/subfolder/c.c");
+}

examples/subfolders/main.c

@@ -24,6 +24,7 @@ void module_a(void);
 void module_b(void);
 void folder_a(void);
 void folder_b(void);
+void folder_c(void);
 
 int main(void)
 {
@@ -34,6 +35,7 @@ int main(void)
     // call functions from subfolder
     folder_a();
     folder_b();
+    folder_c();
 
     return 0;
 }