Why do multiple planes arrive from the same airport at the same time?
This has to do with codesharing or codeshare. Two or more airlines can agree to share the same route and operate it with only one aircraft to reduce cost of operation.
See the WikiPedia article on codeshare 
Here an excerpt:

A codeshare agreement, sometimes simply codeshare, is an aviation business arrangement where two or more airlines share the same flight. A seat can be purchased on one airline but is actually operated by a cooperating airline under a different flight number or code

In the case where multiple flights are listed arriving at the same time from the same departure airports, there will only be one physical airplane serving the connection but under different flight numbers and operators. The different flight numbers and operators are listed to make it easier for passengers to identify their flight if codeshares are being used.
On busy airports or where there is not enough space on the information displays to show all flights of a codeshare, the relation is only shown once with rotating flight numbers.