Sure seemt to be quite unusual, but not at all unbelieveable.

I tried a Goggle search and could only come up with these two covering multiple casualties. Perhaps Greg Biggs or another Flag expert might have greater examples.

There are documented incidents from the Civil War in which as many as six or seven flag bearers in a particular unit were killed or wounded during a single charge or battle.
http://thecenturionpapers.blogspot.com/2006/12/serving-as-flag-bearer.html

…

Captain Richard Waller assumed command of the 8th Florida and he too was killed with the regiment’s colors draped over his shoulders. At Antietam, the entire color guard was killed or wounded.
http://www.civilwar.org/battlefields/antietam/history/antietam-flags/