Pinging Results in Duplicates - CentOS 6 gst, Windows 7 hst

[pgn674]

I am running a new CentOS 6.4 64-bit guest in VirtualBox 4.2.16 r86992 on a Windows 7 Professional 64-bit host. When I use a Bridged Adapter to my ethernet port or WiFi card, then pinging from CentOS works fine (0% packet loss). But, if I use a NAT network adapter, then that's when pings get bad.

The {virtualbox org} forum is telling me "You must be a member for 1 days and have 1 posts before you can post urls.", so there's some {sanatizing} below.

Here is an example of pinging Google:

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$ ping -c 100 {google com}
PING {google com} (173.194.43.41) 56(84) bytes of data.
64 bytes from 10.0.4.2: icmp_seq=1 ttl=55 time=11.5 ms
64 bytes from {1e100 net} (173.194.43.41): icmp_seq=1 ttl=55 time=11.1 ms (DUP!)
64 bytes from 10.0.4.2: icmp_seq=2 ttl=55 time=10.5 ms
64 bytes from {1e100 net} (173.194.43.41): icmp_seq=2 ttl=55 time=11.5 ms (DUP!)
64 bytes from 10.0.4.2: icmp_seq=3 ttl=55 time=15.8 ms
64 bytes from {1e100 net} (173.194.43.41): icmp_seq=3 ttl=55 time=12.6 ms (DUP!)

10.0.4.2 is the NAT interface's gateway.

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$ ip a
...
4: eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 08:00:27:8a:59:04 brd ff:ff:ff:ff:ff:ff
    inet 10.0.4.15/24 brd 10.0.4.255 scope global eth2
    inet6 fe80::a00:27ff:fe8a:5904/64 scope link 
       valid_lft forever preferred_lft forever
...
$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.0.4.0        *               255.255.255.0   U     1      0        0 eth2
default         10.0.4.2        0.0.0.0         UG    0      0        0 eth2

Why is the NAT gateway replying to my pings for Google? Also, switching to NAT results in a bunch of packet loss. Here's an example summary of pinging Wikipedia:

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--- {wikipedia org} ping statistics ---
100 packets transmitted, 61 received, +1 duplicates, 39% packet loss, time 100109ms
rtt min/avg/max/mdev = 25.866/35.572/55.269/9.332 ms

Does anyone know what's going on, or what I can try? I would like to use NAT, because I use the host Windows' VPN to get into my office's network from home, and I use CentOS in my work. I can reply with any logs and configuration details that's asked for.
I have already tried uninstalling and reinstalling VirtualBox, and creating a new VM.