GET requests are specified[0] to be idempotent:

  > Methods can also have the property of "idempotence" in that (aside from
  > error or expiration issues) the side-effects of N > 0 identical
  > requests is the same as for a single request. The methods GET, HEAD,
  > PUT and DELETE share this property.

[0] https://tools.ietf.org/html/rfc2616#section-9.1.2

edit: formatting