I'm assuming this is because of multiple types of bronze? Here's my log:

http://pastebin.com/N62aN6i2

Yea, Most mods rename there materials to something like ic2_bronze, I'll add checks to prevent this.

What version of the mod are you using? A check like this has been added for a long wile now.

I was using the latest 1.7.10 version on Curse, 1.1.0.21

Are you absolutely sure about that?
The line at net.doubledoordev.timber.items.ItemLumberAxe.<init>(ItemLumberAxe.java:71) indicates a really old version, from before the mod was renamed to Lumberjack.
Can you do me a favor and download it fresh from curse?

Timber{1.0} [Timber] (Lumberjack-1.7.10-1.0.0.2.jar)

100% outdated and not the latest version from curse.

EDIT: Fixed Git formatting.