pf: need help to debug "Stack key attach failed"

Hello all,

I recently updated a machine to OpenBSD 4.7 and rewrote the ruleset. Things
seem to be working, though the system message buffer is getting filled with
this error:

pf: stack key attach failed on fxp0: ICMP in wire: (0) xxx.142.100.44:22793
xxx.142.101.241:8 stack: (0) xxx.142.100.44:22793 127.0.0.1:8 0:0 @27, \
existing: ICMP in wire: (0) xxx.142.100.44:22793 xxx.142.101.244:8 stack:
(0) xxx.142.100.44:22793 127.0.0.1:8 0:0 @30

pfctl -vvs rules | grep @27 :

@27 pass in log on fxp0 inet proto icmp from any to xxx.142.101.241
icmp-type echoreq code 0 keep state (if-bound) rdr-to 127.0.0.1


pfctl -vvs rule | grep @30 :

@30 pass in log on fxp0 inet proto icmp from any to xxx.142.101.244
icmp-type echoreq code 0 keep state (if-bound) rdr-to 127.0.0.1


The addresses .241 and .244 are part of a /29 subnet from the ISP and on the
same interface. It looks like they are translated into the same state,
leading to this error.
Can anyone suggest an alternate way to express the rules, or something else
to avoid this error ?

George