Hi,
It depends what you want to do.
You can provide redundancy running two (or more) EX switches in a virtual-chassis
http://www.juniper.net/techpubs/en_US/junos13.2/topics/concept/virtual-chassis-ex4200-components.html
Some EX switches (EX6200, EX8200, EX9200) have redundant routing-engines.
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