diff --git a/src/kernel/unit.c b/src/kernel/unit.c
index 28e95b941..95c689ded 100644
--- a/src/kernel/unit.c
+++ b/src/kernel/unit.c
@@ -494,6 +494,10 @@ attrib_type at_alias = {
     NO_READ
 };
 
+/** remember old unit.no (for the creport, mostly)
+ * if alias is positive, then this unit was a TEMP
+ * if alias is negative, then this unit has been RENUMBERed
+ */
 int ualias(const unit * u)
 {
     attrib *a = a_find(u->attribs, &at_alias);
@@ -1716,7 +1720,7 @@ void renumber_unit(unit *u, int no) {
     uunhash(u);
     if (!ualias(u)) {
         attrib *a = a_add(&u->attribs, a_new(&at_alias));
-        a->data.i = -u->no; // TODO: why is the alias negative? confusing!
+        a->data.i = -u->no;
     }
     u->no = no;
     uhash(u);
diff --git a/src/renumber.c b/src/renumber.c
index 259dfcc2b..de2b5a490 100644
--- a/src/renumber.c
+++ b/src/renumber.c
@@ -36,7 +36,6 @@ void renumber_factions(void)
         attrib *a = a_find(f->attribs, &at_number);
         int want;
         struct renum **rn;
-        faction *old;
 
         if (!a)
             continue;
@@ -45,12 +44,6 @@ void renumber_factions(void)
             ADDMSG(&f->msgs, msg_message("renumber_twice", "id", want));
             continue;
         }
-        old = findfaction(want);
-        if (old) {
-            a_remove(&f->attribs, a);
-            ADDMSG(&f->msgs, msg_message("renumber_inuse", "id", want));
-            continue;
-        }
         if (!faction_id_is_unused(want)) {
             a_remove(&f->attribs, a);
             ADDMSG(&f->msgs, msg_message("renumber_inuse", "id", want));
@@ -151,7 +144,7 @@ int renumber_cmd(unit * u, order * ord)
                 cmistake(u, ord, 114, MSG_EVENT);
                 break;
             }
-            if (findship(i) || findbuilding(i)) {
+            if (findship(i)) {
                 cmistake(u, ord, 115, MSG_EVENT);
                 break;
             }
@@ -180,7 +173,7 @@ int renumber_cmd(unit * u, order * ord)
                 cmistake(u, ord, 114, MSG_EVENT);
                 break;
             }
-            if (findship(i) || findbuilding(i)) {
+            if (findbuilding(i)) {
                 cmistake(u, ord, 115, MSG_EVENT);
                 break;
             }
diff --git a/src/renumber.test.c b/src/renumber.test.c
index 3fabaf4c1..9ab6b1e7b 100644
--- a/src/renumber.test.c
+++ b/src/renumber.test.c
@@ -29,6 +29,25 @@ static void test_renumber_faction(CuTest *tc) {
     test_cleanup();
 }
 
+static void test_renumber_faction_duplicate(CuTest *tc) {
+    unit *u;
+    faction *f, *f2;
+    int no;
+    const struct locale *lang;
+
+    test_setup_ex(tc);
+    f2 = test_create_faction(0);
+    u = test_create_unit(f = test_create_faction(0), test_create_region(0, 0, 0));
+    no = f->no;
+    lang = f->locale;
+    u->thisorder = create_order(K_NUMBER, lang, "%s %s", LOC(lang, parameters[P_FACTION]), itoa36(f2->no));
+    renumber_cmd(u, u->thisorder);
+    renumber_factions();
+    CuAssertPtrNotNull(tc, test_find_messagetype(f->msgs, "renumber_inuse"));
+    CuAssertIntEquals(tc, no, u->faction->no);
+    test_cleanup();
+}
+
 static void test_renumber_building(CuTest *tc) {
     unit *u;
     int uno, no;
@@ -46,6 +65,26 @@ static void test_renumber_building(CuTest *tc) {
     test_cleanup();
 }
 
+static void test_renumber_building_duplicate(CuTest *tc) {
+    unit *u;
+    faction *f;
+    int uno, no;
+    const struct locale *lang;
+
+    test_setup_ex(tc);
+    u = test_create_unit(f = test_create_faction(0), test_create_region(0, 0, 0));
+    u->building = test_create_building(u->region, 0);
+    uno = u->building->no;
+    u->building = test_create_building(u->region, 0);
+    no = u->building->no;
+    lang = f->locale;
+    u->thisorder = create_order(K_NUMBER, lang, "%s %s", LOC(lang, parameters[P_BUILDING]), itoa36(uno));
+    renumber_cmd(u, u->thisorder);
+    CuAssertPtrNotNull(tc, test_find_messagetype(f->msgs, "error115"));
+    CuAssertIntEquals(tc, no, u->building->no);
+    test_cleanup();
+}
+
 static void test_renumber_ship(CuTest *tc) {
     unit *u;
     int uno, no;
@@ -63,6 +102,26 @@ static void test_renumber_ship(CuTest *tc) {
     test_cleanup();
 }
 
+static void test_renumber_ship_duplicate(CuTest *tc) {
+    unit *u;
+    faction *f;
+    int uno, no;
+    const struct locale *lang;
+
+    test_setup_ex(tc);
+    u = test_create_unit(f = test_create_faction(0), test_create_region(0, 0, 0));
+    u->ship = test_create_ship(u->region, 0);
+    uno = u->ship->no;
+    u->ship = test_create_ship(u->region, 0);
+    no = u->ship->no;
+    lang = f->locale;
+    u->thisorder = create_order(K_NUMBER, lang, "%s %s", LOC(lang, parameters[P_SHIP]), itoa36(uno));
+    renumber_cmd(u, u->thisorder);
+    CuAssertPtrNotNull(tc, test_find_messagetype(f->msgs, "error115"));
+    CuAssertIntEquals(tc, no, u->ship->no);
+    test_cleanup();
+}
+
 static void test_renumber_unit(CuTest *tc) {
     unit *u;
     int uno, no;
@@ -76,7 +135,62 @@ static void test_renumber_unit(CuTest *tc) {
     u->thisorder = create_order(K_NUMBER, lang, "%s %s", LOC(lang, parameters[P_UNIT]), itoa36(uno));
     renumber_cmd(u, u->thisorder);
     CuAssertIntEquals(tc, uno, u->no);
-    CuAssertIntEquals(tc, -no, ualias(u)); // TODO: why is ualias negative?
+    CuAssertIntEquals(tc, -no, ualias(u));
+    test_cleanup();
+}
+
+static void test_renumber_unit_duplicate(CuTest *tc) {
+    unit *u, *u2;
+    faction *f;
+    int no;
+    const struct locale *lang;
+
+    test_setup_ex(tc);
+    u = test_create_unit(f = test_create_faction(0), test_create_region(0, 0, 0));
+    no = u->no;
+    u2 = test_create_unit(f, u->region);
+    lang = f->locale;
+    u->thisorder = create_order(K_NUMBER, lang, "%s %s", LOC(lang, parameters[P_UNIT]), itoa36(u2->no));
+    renumber_cmd(u, u->thisorder);
+    CuAssertIntEquals(tc, no, u->no);
+    CuAssertPtrNotNull(tc, test_find_messagetype(f->msgs, "error115"));
+    CuAssertIntEquals(tc, 0, ualias(u));
+    test_cleanup();
+}
+
+static void test_renumber_unit_limit(CuTest *tc) {
+    unit *u;
+    faction *f;
+    int no;
+    const struct locale *lang;
+
+    test_setup_ex(tc);
+    u = test_create_unit(f = test_create_faction(0), test_create_region(0, 0, 0));
+    no = u->no;
+    lang = f->locale;
+    u->thisorder = create_order(K_NUMBER, lang, "%s 10000", LOC(lang, parameters[P_UNIT]));
+    renumber_cmd(u, u->thisorder);
+    CuAssertIntEquals(tc, no, u->no);
+    CuAssertPtrNotNull(tc, test_find_messagetype(f->msgs, "error114"));
+    CuAssertIntEquals(tc, 0, ualias(u));
+    test_cleanup();
+}
+
+static void test_renumber_unit_invalid(CuTest *tc) {
+    unit *u;
+    faction *f;
+    int no;
+    const struct locale *lang;
+
+    test_setup_ex(tc);
+    u = test_create_unit(f = test_create_faction(0), test_create_region(0, 0, 0));
+    no = u->no;
+    lang = f->locale;
+    u->thisorder = create_order(K_NUMBER, lang, "%s TEMP", LOC(lang, parameters[P_UNIT]));
+    renumber_cmd(u, u->thisorder);
+    CuAssertIntEquals(tc, no, u->no);
+    CuAssertPtrNotNull(tc, test_find_messagetype(f->msgs, "error116"));
+    CuAssertIntEquals(tc, 0, ualias(u));
     test_cleanup();
 }
 
@@ -84,8 +198,14 @@ CuSuite *get_renumber_suite(void)
 {
     CuSuite *suite = CuSuiteNew();
     SUITE_ADD_TEST(suite, test_renumber_unit);
+    SUITE_ADD_TEST(suite, test_renumber_unit_limit);
+    SUITE_ADD_TEST(suite, test_renumber_unit_duplicate);
+    SUITE_ADD_TEST(suite, test_renumber_unit_invalid);
     SUITE_ADD_TEST(suite, test_renumber_building);
+    SUITE_ADD_TEST(suite, test_renumber_building_duplicate);
     SUITE_ADD_TEST(suite, test_renumber_ship);
+    SUITE_ADD_TEST(suite, test_renumber_ship_duplicate);
     SUITE_ADD_TEST(suite, test_renumber_faction);
+    SUITE_ADD_TEST(suite, test_renumber_faction_duplicate);
     return suite;
 }