diff --git a/common/list.c b/common/list.c
index 45efc16f2..310296d87 100644
--- a/common/list.c
+++ b/common/list.c
@@ -53,3 +53,13 @@ void list_cat(list_t *list, list_t *source) {
 void list_sort(list_t *list, int compare(const void *left, const void *right)) {
 	qsort(list->items, list->length, sizeof(void *), compare);
 }
+
+int list_seq_find(list_t *list, int compare(const void *item, const void *data), const void *data) {
+	for (int i = 0; i < list->length; i++) {
+		void *item = list->items[i];
+		if (compare(item, data) == 0) {
+			return i;
+		}
+	}
+	return -1;
+}
diff --git a/include/config.h b/include/config.h
index c93f9cafc..82aa71bfb 100644
--- a/include/config.h
+++ b/include/config.h
@@ -102,6 +102,8 @@ char *do_var_replacement(char *str);
 void apply_output_config(struct output_config *oc, swayc_t *output);
 void free_output_config(struct output_config *oc);
 
+int workspace_output_cmp_workspace(const void *a, const void *b);
+
 /**
  * Global config singleton.
  */
diff --git a/include/list.h b/include/list.h
index aff6800f0..90d0ad36d 100644
--- a/include/list.h
+++ b/include/list.h
@@ -15,5 +15,8 @@ void list_del(list_t *list, int index);
 void list_cat(list_t *list, list_t *source);
 // See qsort
 void list_sort(list_t *list, int compare(const void *left, const void *right));
+// Return index for first item in list that returns 0 for given compare
+// function or -1 if none matches.
+int list_seq_find(list_t *list, int compare(const void *item, const void *cmp_to), const void *cmp_to);
 
 #endif
diff --git a/include/stringop.h b/include/stringop.h
index febbbaba5..bb681bcdb 100644
--- a/include/stringop.h
+++ b/include/stringop.h
@@ -14,6 +14,9 @@ char *strip_whitespace(char *str);
 char *strip_comments(char *str);
 void strip_quotes(char *str);
 
+// strcmp that also handles null pointers.
+int lenient_strcmp(char *a, char *b);
+
 // Simply split a string with delims, free with `free_flat_list`
 list_t *split_string(const char *str, const char *delims);
 void free_flat_list(list_t *list);
diff --git a/sway/commands.c b/sway/commands.c
index 173f0f531..7f24f5ab0 100644
--- a/sway/commands.c
+++ b/sway/commands.c
@@ -1318,9 +1318,15 @@ static struct cmd_results *cmd_workspace(int argc, char **argv) {
 				return error;
 			}
 			struct workspace_output *wso = calloc(1, sizeof(struct workspace_output));
-			sway_log(L_DEBUG, "Assigning workspace %s to output %s", argv[0], argv[2]);
 			wso->workspace = strdup(argv[0]);
 			wso->output = strdup(argv[2]);
+			int i = -1;
+			if ((i = list_seq_find(config->workspace_outputs, workspace_output_cmp_workspace, wso)) != -1) {
+				struct workspace_output *old = config->workspace_outputs->items[i];
+				free(old); // workspaces can only be assigned to a single output
+				list_del(config->workspace_outputs, i);
+			}
+			sway_log(L_DEBUG, "Assigning workspace %s to output %s", argv[0], argv[2]);
 			list_add(config->workspace_outputs, wso);
 			if (!config->reading) {
 				// TODO: Move workspace to output. (dont do so when reloading)
diff --git a/sway/config.c b/sway/config.c
index 725dedd2b..2fceb6e21 100644
--- a/sway/config.c
+++ b/sway/config.c
@@ -375,3 +375,11 @@ char *do_var_replacement(char *str) {
 	}
 	return str;
 }
+
+// the naming is intentional (albeit long): a workspace_output_cmp function
+// would compare two structs in full, while this method only compares the
+// workspace.
+int workspace_output_cmp_workspace(const void *a, const void *b) {
+	const struct workspace_output *wsa = a, *wsb = b;
+	return lenient_strcmp(wsa->workspace, wsb->workspace);
+}
diff --git a/sway/stringop.c b/sway/stringop.c
index 8d6cac2fd..fe5a97ca1 100644
--- a/sway/stringop.c
+++ b/sway/stringop.c
@@ -74,6 +74,19 @@ void strip_quotes(char *str) {
 	*end = '\0';
 }
 
+// strcmp that also handles null pointers.
+int lenient_strcmp(char *a, char *b) {
+	if (a == b) {
+		return 0;
+	} else if (!a) {
+		return -1;
+	} else if (!b) {
+		return 1;
+	} else {
+		return strcmp(a, b);
+	}
+}
+
 list_t *split_string(const char *str, const char *delims) {
 	list_t *res = create_list();
 	char *copy = strdup(str);