Index: src/includes/admin/classes/class-bbp-converter-base.php
===================================================================
--- src/includes/admin/classes/class-bbp-converter-base.php	(revision 6649)
+++ src/includes/admin/classes/class-bbp-converter-base.php	(working copy)
@@ -477,12 +477,27 @@
 							/** New user **************************************/
 
 							case 'user':
-								if ( username_exists( $insert_post['user_login'] ) ) {
-									$insert_post['user_login'] = 'imported_' . $insert_post['user_login'];
-								}
 
-								if ( email_exists( $insert_post['user_email'] ) ) {
-									$insert_post['user_email'] = 'imported_' . $insert_post['user_email'];
+								$old_user_login       = $insert_post['user_login'];
+								$sanitized_user_login = sanitize_user( $insert_post['user_login'], true );
+
+								// Both return the user's ID on success, and false on failure.
+								$user_exists  = username_exists( $sanitized_user_login );
+								$email_exists = email_exists( $insert_post['user_email'] );
+
+								if ( $user_exists !== false && $email_exists !== false
+									&& $user_exists === $email_exists
+								) {
+									// Update an existing user instead of inserting a new one.
+									$insert_post['ID'] = $user_exists;
+								} else {
+									if ( $user_exists ) {
+										$insert_post['user_login'] = 'imported_' . $sanitized_user_login;
+									}
+
+									if ( $email_exists ) {
+										$insert_post['user_email'] = 'imported_' . $insert_post['user_email'];
+									}
 								}
 
 								if ( empty( $insert_post['user_pass'] ) ) {
@@ -489,6 +504,12 @@
 									$insert_post['user_pass'] = '';
 								}
 
+								if ( $sanitized_user_login !== $old_user_login
+									&& empty( $insert_post['display_name'] )
+								) {
+									$insert_post['display_name'] = $old_user_login;
+								}
+
 								$post_id = wp_insert_user( $insert_post );
 
 								if ( is_numeric( $post_id ) ) {
@@ -502,6 +523,8 @@
 										}
 									}
 								}
+
+								unset( $old_user_login, $sanitized_user_login, $user_exists, $email_exists );
 								break;
 
 							/** New Topic-Tag *********************************/