Novas verilog question

[skyworld_cy]

hi,
I'm working with Novas and ncverilog together to dump waveforms. It is strange that the log always shows this message:

*** Registering Novas Verilog system tasks...
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpfile has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpvarsToFile has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpvars has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbSwitchDumpfile has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbAutoSwitchDumpfile has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpon has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpoff has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpflush has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumplimit has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpStrength has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpMem has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpMemNow has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDisplay has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbSuppress has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpMemInFile has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpMemInScope has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpFinish has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbSubstituteHier has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpPSL has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpPSLon has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpPSLoff has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpSVA has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpSVAon has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpSVAoff has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpMDAInScope has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpMDAOnChange has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpMDA has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpMDANow has already been registered.
ERROR: TF TFTHRE
The user defined task or function $fsdbDumpPattern has already been registered

Can anybody help me to find the reason why this would happen? Thanks.