I had similar situation, to send reminder to the assigned users. In button click you can use below code, which will send single mail to single user who are assigned to more than one task.
What happening here actually i am saving the user information who is already informed, so that it should not receive more than one mail on single button click.
StringBuilder nusers = new StringBuilder();
SPFieldUserValueCollection userVals = (SPFieldUserValueCollection)ListItem[SPBuiltInFieldId.AssignedTo];
foreach (SPFieldUserValue userVal in userVals)
SPUser user = userVal.User;
string nAssignedTo = user.Name.ToString();
//do your stuff...
Try out the code and let me know if you have any doubt.