Thread: error: dereferencing pointer to incomplete type

What the does this error mean its so frustrating.
basically i have a struct created in main

Code:

struct Ethfrm_t
{
	struct FCfrm_t* ptr;
	struct FIPfrm_t* FIPptr;
	unsigned char DestEthAdr[6];
	unsigned char SrcEthAdr[6];
	UINT32  VLAN_HdrSave;
	UINT32  reserved;
	UINT32  pad;
	unsigned char Ethbuffer[BUF_SIZE];
}  __attribute__ ((packed));

then i have
typedef struct Ethfrm_t Ethfrm_t; On a header file on the top of most header files

but when i made a functions file *.c When i use that Struct i get that warning all the time

Code:

void printEthfrm(Ethfrm_t * Ethfrm){
	unsigned char* etherhead = Ethfrm->Ethbuffer;                ////right here
 	printf("EtherType error ");
}

Okay well this is retarded but now i put it in a header

Code:

typedef struct Ethfrm_t
{
	struct FCfrm_t* ptr;
	struct FIPfrm_t* FIPptr;
	unsigned char DestEthAdr[6];
	unsigned char SrcEthAdr[6];
	UINT32  VLAN_HdrSave;
	UINT32  reserved;
	UINT32  pad;
	unsigned char Ethbuffer[BUF_SIZE];
}  __attribute__ ((packed)) Ethfrm_t;

I get this error
server.h:11: error: redefinition of ‘struct Ethfrm_t’
server.h:20: error: redefinition of typedef ‘Ethfrm_t’
server.h:20: error: previous declaration of ‘Ethfrm_t’ was here
iono what its problem is but i doesnt like me putting Ethfrm_t on the top and bottom????? Y