diff --git a/CMakeLists.txt b/CMakeLists.txt
index 074fb768..df8a75dc 100755
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -292,14 +292,11 @@ else()
 endif()
 
 get_target_property(INS drogon INCLUDE_DIRECTORIES)
-set(TMP_INS "")
+list(APPEND INS "${INSTALL_INCLUDE_DIR}")
+list(REMOVE_DUPLICATES INS)
 set(INS_STRING "")
 foreach(loop_var ${INS})
-    if(TMP_INS MATCHES ";${loop_var};")
-    else()
-        set(TMP_INS ";${loop_var};${TMP_INS}")
-        set(INS_STRING "${INS_STRING} -I${loop_var}")
-    endif()
+    set(INS_STRING "${INS_STRING} -I${loop_var}")
 endforeach(loop_var)
 
 set(INCLUDING_DIRS ${INS_STRING})