diff --git a/src/script/script.h b/src/script/script.h
--- a/src/script/script.h
+++ b/src/script/script.h
@@ -437,6 +437,7 @@
     }
 
     CScript &operator+=(const CScript &b) {
+        reserve(size() + b.size());
         insert(end(), b.begin(), b.end());
         return *this;
     }
diff --git a/src/test/script_tests.cpp b/src/test/script_tests.cpp
--- a/src/test/script_tests.cpp
+++ b/src/test/script_tests.cpp
@@ -3221,4 +3221,23 @@
     }
 }
 
+BOOST_AUTO_TEST_CASE(script_can_append_self) {
+    CScript s, d;
+
+    s = ScriptFromHex("00");
+    s += s;
+    d = ScriptFromHex("0000");
+    BOOST_CHECK(s == d);
+
+    // check doubling a script that's large enough to require reallocation
+    static const char hex[] =
+        "04678afdb0fe5548271967f1a67130b7105cd6a828e03909a67962e0ea1f61deb649f6"
+        "bc3f4cef38c4f35504e51ec112de5c384df7ba0b8d578a4c702b6bf11d5f";
+    s = CScript() << ParseHex(hex) << OP_CHECKSIG;
+    d = CScript() << ParseHex(hex) << OP_CHECKSIG << ParseHex(hex)
+                  << OP_CHECKSIG;
+    s += s;
+    BOOST_CHECK(s == d);
+}
+
 BOOST_AUTO_TEST_SUITE_END()