Error in adding new class members

Hello, I cannot add users to a class - trying to do so leads to an 'unexpected error' page, and the error log says: PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '112-112-og_access:node' for key 'PRIMARY': INSERT INTO {node_access} (nid, realm, gid, grant_view, grant_update, grant_delete) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5), (:db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9, :db_insert_placeholder_10, :db_insert_placeholder_11), (:db_insert_placeholder_12, :db_insert_placeholder_13, :db_insert_placeholder_14, :db_insert_placeholder_15, :db_insert_placeholder_16, :db_insert_placeholder_17); Array ( [:db_insert_placeholder_0] => 112 [:db_insert_placeholder_1] => content_access_rid [:db_insert_placeholder_2] => 3 [:db_insert_placeholder_3] => 1 [:db_insert_placeholder_4] => 0 [:db_insert_placeholder_5] => 0 [:db_insert_placeholder_6] => 112 [:db_insert_placeholder_7] => og_access:node [:db_insert_placeholder_8] => 112 [:db_insert_placeholder_9] => 1 [:db_insert_placeholder_10] => 0 [:db_insert_placeholder_11] => 0 [:db_insert_placeholder_12] => 112 [:db_insert_placeholder_13] => og_access:node [:db_insert_placeholder_14] => 112 [:db_insert_placeholder_15] => 1 [:db_insert_placeholder_16] => 0 [:db_insert_placeholder_17] => 0 ) in node_access_write_grants() (line 3546 of /srv/bindings/56ffcc1189d741cba719f6c6cef2f1bb/code/modules/node/node.module). I'd be extremely grateful for your help. Thanks, Harriet