Calendar issue : REsechduled meeting is displayed as a separate meeting

We have a user who Re-scheduled one meeting from 6:15 to 6:30 AM it is strange that there are two lines in her calendar with the old meeting and the new one. The same problem appears in the calendar of recipient who was invited to this meeting.

There is another problem, when sender reschedules the meeting there are some people who receive emails with update, but there is no option to accept the new hour of meeting, so they appeare in sender’s calendar with the status “no response” (and they cannot accept the new hour), nevertheless, some people could accept new hour of the meeting. Pic attached below:

http://www.picvalley.net/v.php?p=u/1508/10987129659991777531320083404Q79wsgwKmxg136PI94un.JPG

Please advise