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Grails has built-in support for handling duplicate form submissions using the "Synchronizer Token Pattern". To get started you define a token on the link:{gspTagsRef}form.html[form] tag:

[source,xml]
----
<g:form useToken="true" ...>
----

Then in your controller code you can use the link:{controllersRef}withForm.html[withForm] method to handle valid and invalid requests:

[source,groovy]
----
withForm {
   // good request
}.invalidToken {
   // bad request
}
----

If you only provide the link:{controllersRef}withForm.html[withForm] method and not the chained `invalidToken` method then by default Grails will store the invalid token in a `flash.invalidToken` variable and redirect the request back to the original page. This can then be checked in the view:

[source,xml]
----
<g:if test="${flash.invalidToken}">
  Don't click the button twice!
</g:if>
----

WARNING: The link:{controllersRef}withForm.html[withForm] tag makes use of the link:{controllersRef}session.html[session] and hence requires session affinity or clustered sessions if used in a cluster.
