При касте шипов на цель, на себя (при уже действующих шипах, более сильных) должно появляться сообщение - уже действует более сильное заклинание. И соответственно не должно перебивать более сильную версию как по ранку спелла, так и по силе (которая зависит от спд кастера).

Пруф
http://www.wowwiki.com/Thorns
"When refreshing Thorns on yourself, you may sometimes get the error message, 'A more powerful spell is already active.' This is understandable if the initial buff was cast by a different druid with a higher rank of the spell or talents to increase its potency, but may be confusing when it was you who cast the spell. The reason for this error message is that Thorns benefits from the druid's spell power at the time it was cast, not at the time of the hit. Thus, casting Thorns on yourself while at a higher spell damage (such as with a trinket activated) will make it impossible to refresh the buff without first right-clicking it off if your spell power has dropped down. Some buffs and items may temporarily increase spell power, causing this to occur. If using similar items or abilities with short cooldowns, it would be best to max out spell power before casting Thorns to get the maximum effect out of it. "