Sample chater of REST book , which contain a nice discussion of why Idempotence is important - http://www.infoq.com/resource/articles/richardson-ruby-restful-ws/en/resources/04.pdf

New Acid:
    * A – Associative
    * C – Commutative
    * I – Idempotent
    * D - Distributed

http://www.eaipatterns.com/ramblings/68_acid.html

Idempotency patterns - http://jonathan-oliver.blogspot.com/2010/04/idempotency-patterns.html

Detailed discussion about how to design Idempotence - http://queue.acm.org/detail.cfm?id=2187821

Discuss about clock in distributed environment - http://basho.com/clocks-are-bad-or-welcome-to-distributed-systems/

Idempotent operations - https://levelup.gitconnected.com/idempotent-operations-b1d4b9b76417