Add Travis check for duplicate includes

This enforces parts of the project header include guidelines (added by @sipa in #10575).

contrib/devtools/lint-includes.sh

@@ -0,0 +1,43 @@
+#!/bin/bash
+#
+# Copyright (c) 2018 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#
+# Check for duplicate includes.
+
+filter_suffix() {
+    git ls-files | grep -E "^src/.*\.${1}"'$' | grep -Ev "/(leveldb|secp256k1|univalue)/"
+}
+
+EXIT_CODE=0
+for HEADER_FILE in $(filter_suffix h); do
+    DUPLICATE_INCLUDES_IN_HEADER_FILE=$(grep -E "^#include " < "${HEADER_FILE}" | sort | uniq -d)
+    if [[ ${DUPLICATE_INCLUDES_IN_HEADER_FILE} != "" ]]; then
+        echo "Duplicate include(s) in ${HEADER_FILE}:"
+        echo "${DUPLICATE_INCLUDES_IN_HEADER_FILE}"
+        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)
+    if [[ ${DUPLICATE_INCLUDES_IN_CPP_FILE} != "" ]]; then
+        echo "Duplicate include(s) in ${CPP_FILE}:"
+        echo "${DUPLICATE_INCLUDES_IN_CPP_FILE}"
+        echo
+        EXIT_CODE=1
+    fi
+done
+exit ${EXIT_CODE}