The OP is probably a new hunter and not from a state with bears. OR the OP is actually wanting to shoot a black bear if the OP stumbles upon one while duck hunting. Black bears are not a problem. The amount of black bear attacks in general is crazy low for the number of human to bear encounters there are. The amount of black bear attacks on duck hunters is zero, or as close to it as possible. Preparing for a black bear attack while duck hunting would be like wearing a seat belt while walking. It is unnecessary.
There was a grizzly bear attack while duck hunting in Alaska. It is the only bear attack I can find while duck hunting. So the odds are what, 1 in a trillion of getting attacked by a bear while duck hunting? In the Alaska case, the duck hunters shot the griz with 12 gauge 3" #4 steel shot. It was slowed by the 2nd shot and severely slowed by the 3rd shot. The friend then fired 3 more shots of 12 gauge 3.5" #4 steel and the bear went into a death roll on the ground. They then reloaded and fired many more shots...
The hunters say they carried slugs in case they encountered a rogue grizzly, but there was no time to grab them.
So in a self defense situation, any duck loads in the gun would work at close range.
If the OP is indeed wanting to shoot a bear while duck hunting given a chance, you have options, but using #BB TSS would be illegal since you need #1 buckshot or larger by law.