Fixed erros when adding duplicate tags/flat terms. (#4487)

If we get an error saying we have a duplicate tag/flat term (tag may have been added by other used since last time we fetched), we search the server for the new tag and we add it to our list.

editor/components/post-taxonomies/flat-term-selector.js

@@ -106,8 +106,13 @@ class FlatTermSelector extends Component {
 				.then( resolve, ( xhr ) => {
 					const errorCode = xhr.responseJSON && xhr.responseJSON.code;
 					if ( errorCode === 'term_exists' ) {
-						return new Model( { id: xhr.responseJSON.data } )
-							.fetch().then( resolve, reject );
+						// search the new category created since last fetch
+						this.addRequest = new Model().fetch(
+							{ data: { ...DEFAULT_QUERY, search: termName } }
+						);
+						return this.addRequest.then( searchResult => {
+							resolve( find( searchResult, result => result.name === termName ) );
+						}, reject );
 					}
 					reject( xhr );
 				} );