Double-submit of TokenSessionStoreInterceptor broken since 2.5.16

With recent fixes to the Showcase application it was discovered that the 3rd Token example (for TokenSessionStoreInterceptor) is failing upon the double-submit with Struts versions 2.5.16 - 2.5.20 (and 2.5.21 snapshot). Steps to reproduce:
1) Launch Struts 2.5.21-snapshot showcase application
2) Open the Examples, Token, Example 3.
3) Click on submit (wait for completion), back button in browser, then submit again.
4) Application container returns a response code 500.

The issue appears to have been introduced with fixes for WW-4873. A proposed fix for this issue will be provided in a PR shortly.