ping only returns once

[Jackrabbitt Slim <linux graffitilandscape com>]

I have been mucking around with my linux box trying to get two network cards
to do IP forwarding.  I seem to have created this scenario:

1.  I can ping external PCs with consistency.
2.  Other PCs can ping me fine also.
3.  When I ping myself (either network card) I get one reply and the rest
are not responded to.  (e.g. 10 pings gives 90% failed).

Anybody have any idea what might be the cause of this?