Error: L6200E: Symbol igmp_is_member multiply defined (by at_igmp.o and net_config.o)

Has anyone encountered this problem and do you have a solution? I've included info on the IDE below and am using Keil runtime library V3.1. I'm trying to build a simple multicast application.

Thanks,
Mike

IDE-Version:
µVision V4.21.0.0
Copyright (C) 2011 ARM Ltd and ARM Germany GmbH. All rights reserved.

Tool Version Numbers:
Toolchain: MDK-ARM Professional Version: 4.21
Middleware: RL-ARM Real-Time Library Version V4.12
Toolchain Path: BIN40\
C Compiler: Armcc.Exe V4.1.0.713
Assembler: Armasm.Exe V4.1.0.713
Linker/Locator: ArmLink.Exe V4.1.0.713
Librarian: ArmAr.Exe V4.1.0.713
Hex Converter: FromElf.Exe V4.1.0.713
CPU DLL: SARMCM3.DLL V4.21
Dialog DLL: DLM.DLL V1.22
Target DLL: BIN\UL2CM3.DLL V1.90
Dialog DLL: TLM.DLL V1.21