Refine travis check for duplicate includes

This partially reverts commit c36b720d00.

contrib/devtools/lint-includes.sh

@@ -19,17 +19,6 @@ for HEADER_FILE in $(filter_suffix h); do
         echo
         EXIT_CODE=1
     fi
-    CPP_FILE=${HEADER_FILE/%\.h/.cpp}
-    if [[ ! -e $CPP_FILE ]]; then
-        continue
-    fi
-    DUPLICATE_INCLUDES_IN_HEADER_AND_CPP_FILES=$(grep -hE "^#include " <(sort -u < "${HEADER_FILE}") <(sort -u < "${CPP_FILE}") | grep -E "^#include " | sort | uniq -d)
-    if [[ ${DUPLICATE_INCLUDES_IN_HEADER_AND_CPP_FILES} != "" ]]; then
-        echo "Include(s) from ${HEADER_FILE} duplicated in ${CPP_FILE}:"
-        echo "${DUPLICATE_INCLUDES_IN_HEADER_AND_CPP_FILES}"
-        echo
-        EXIT_CODE=1
-    fi
 done
 for CPP_FILE in $(filter_suffix cpp); do
     DUPLICATE_INCLUDES_IN_CPP_FILE=$(grep -E "^#include " < "${CPP_FILE}" | sort | uniq -d)

doc/developer-notes.md

@@ -569,8 +569,7 @@ Source code organization
   - *Rationale*: Shorter and simpler header files are easier to read, and reduce compile time
 
 - Every `.cpp` and `.h` file should `#include` every header file it directly uses classes, functions or other
-  definitions from, even if those headers are already included indirectly through other headers. One exception
-  is that a `.cpp` file does not need to re-include the includes already included in its corresponding `.h` file.
+  definitions from, even if those headers are already included indirectly through other headers.
 
   - *Rationale*: Excluding headers because they are already indirectly included results in compilation
     failures when those indirect dependencies change. Furthermore, it obscures what the real code