Added code to database_api.php which solved the problem for now:

function db_insert_id($p_table = null) {
    global $g_db;

if ( isset($p_table) && db_isoci8() ) {
$query = "SELECT seq".$p_table.".currval FROM DUAL";
$result = db_query( $query );
return db_result($result);
}
if ( isset($p_table) && db_is_pgsql() ) {
$query = "SELECT currval('".$p_table."_id_seq')";
$result = db_query( $query );
return db_result($result);
}
return $g_db->Insert_ID( );
}

and added function:

# Check is the database is OCI
function db_is_oci8() {
    $t_db_type = config_get( 'db_type' );

    switch( $t_db_type ) {
        case 'oci8':
            return true;
    }

    return false;
}

This solves the issue for now. SEQUENCE.CURRVAL is private to the DB session. Only when more users share a connection (is this the case in Mantis?) this can cause problems when 2 users make an insert in the same table at almost the same time.

The same solution for the problem just coded another way.
I created an _insertID() function within the OCI wrapper.

diff -rupN mantisbt-1.1.7/core/adodb/drivers/adodb-oci8.inc.php mantisbt-1.1.7_zce/core/adodb/drivers/adodb-oci8.inc.php
--- mantisbt-1.1.7/core/adodb/drivers/adodb-oci8.inc.php 2009-04-20 09:14:00.000000000 +0200
+++ mantisbt-1.1.7_zce/core/adodb/drivers/adodb-oci8.inc.php 2009-05-13 11:24:18.163250000 +0200
@@ -87,7 +87,8 @@ class ADODB_oci8 extends ADOConnection {
var $useDBDateFormatForTextInput=false;
var $datetime = false; // MetaType('DATE') returns 'D' (datetime==false) or 'T' (datetime == true)
var $_refLOBs = array();

• var $hasInsertID = true;

• // var $ansiOuter = true; // if oracle9

  function ADODB_oci8()
  @@ -95,6 +96,15 @@ class ADODB_oci8 extends ADOConnection {
  $this->_hasOCIFetchStatement = ADODB_PHPVER >= 0x4200;
  if (defined('ADODBEXTENSION')) $this->rsPrefix .= 'ext';
  }

• function _insertid($table)
• {
• $seq = "SEQ_".$table;
• $sql = "SELECT $seq.currval from dual";
• $id = ADOConnection::GetOne($sql);
• return $id;
• }

• / Function &MetaColumns($table) added by smondino@users.sourceforge.net/
  function &MetaColumns($table)
  diff -rupN mantisbt-1.1.7/core/database_api.php mantisbt-1.1.7_zce/core/database_api.php
  --- mantisbt-1.1.7/core/database_api.php 2009-04-20 09:14:00.000000000 +0200
  +++ mantisbt-1.1.7_zce/core/database_api.php 2009-05-13 11:12:42.069500000 +0200
  @@ -256,7 +256,7 @@
  $result = db_query( $query );
  return db_result($result);
  }

• return $g_db->Insert_ID( );

• return $g_db->Insert_ID($p_table);
  }

  --------------------

If I get the time I try to figure out if there is a problem with two simultaneous queries and CURRVAL.

Would you have a chance to test if problem still exists using oracle branch in 0013227 (https://github.com/dregad/mantisbt/commits/oracle), and submit a revised patch/pull request against that if it's the case ?