list GetIndex(list source, integer element)
{
    integer i;
    integer len = llGetListLength(source);
    list indexes;
    for(i= 0; i < len; i++)
    {
        if(llList2Integer(source, i) == element)
        {
            indexes += [i];
        }
    }
    return(indexes);
}

default
{
    state_entry()
    {
        list test = [1,1,2,3,1,4,5,3,1,4,7,1,4,2,6,2,1,5,7,4,2,3,6];
        list indx = GetIndex(test, 1);
        llOwnerSay("\ntest = [" + llDumpList2String(test, ",  ") + "]\nindx = [" + llDumpList2String(indx, ",  ") + "]");
    }
}