fsck and 1k block sizes

[utzoo!decvax!ucbvax!npois!npoiv!harpo!esquire!nrh]

I'm trying to bring up 2.8bsd on an 11/70 with SI 9400 controllers
controlling CDC 9766 300 MB drives.  

I notice that fsck seems to work fine when run on the block devices, but
barfs when used with the raw devices.  The symptom is a list of 
messages of the form:
	601 DUP I=283
	604 DUP I=284
	607 DUP I=285
	610 DUP I=285
	.... (all further I='s are 285, and the block numbers are 3 apart)
	634 DUP I=285
	EXCESSIVE DUP BLKS I=285 
	CONTINUE? 

Oddly, this works fine on the block devices.  Naturally, I've made sure
that all reads are happening at 1k block sizes.  Has anyone else come
across (and hopefully fixed) this problem?