var groupings = myList.GroupBy(listElement => listElement.Key);
var groupingsSafe = groupings.ToDictionary(g => g.Key, g => g.ToList());
foreach(var groupoing in groupingsSafe)
{
   ...
}

var groupings = myList.GroupBy(listElement => listElement.Key);
var groupingsSafe = groupings.ToDictionary(g => g.Key, g => g.ToList());
foreach(var groupoing in groupingsSafe)
{
   ...
}