var firstList = project.Lists["list_1"];
var secondList = project.Lists["list_2"];
lock(SyncObjects.ListSyncer)
{
for(int i=0; i < firstList.Count; i++)
{
var str1 = firstList[i];
for(int j=secondList.Count-1; j >= 0; j--)
{
var str2 = secondList[j];
if (str1==str2)
{
secondList.RemoveAt(j);
}
}
}
}