HMR assistance

If I have the following HMR to add a diversion header to add a diversion header if a history info header exists, and I add another HMR for all Cisco phones to add a diversion header (for call masking purposes), when it gets to the second HMR, will the SBC add a second diversion header even if there is already one there?  If so, can a check be done to see if diversion header exists before adding one?

 

header-rule

name checkHistoryExists

header-name History-Info

action                                  store

comparison-type pattern-rule

msg-type request

methods INVITE

match-value

new-value

element-rule

name StoreRequestURI

parameter-name

type                                    uri-user

action                                  store