Your first consideration flow control we provide sequence number to our frames so that duplicate is avoided is an "in air" statement i consider. Flow control simply means 'Sender must not have to send data faster than receiver can receive' and to achieve this data link layer and Transport layer uses concept of sliding window protocols. Sliding window protocols also helps to achieve Error Control. Providing sequence number to frames at data link layer would help not only to discard duplicate frame but to take care inorder delivery and to figure out missing frames.to take care about Flow Control at data link layer there are other ways too , than Sliding window protocols , which won't use the concept of sequence numbering.
Now come to your question , Why doesn't Ethernet frame(802.3) uses this sequencing numbering concept?
LAN protocols don't even use flow control mechanism and error control mechanism because LAN means network in shorter distance , and in shorter distance there are less chances of error and it's better to don't kill efficiency of network by providing extra facilities ,which are of less priority.
Ethernet uses the concept of 'Pause Frame' to control flow.
Although HDLC and other wireless protocols uses flow control ARQ policies because in wireless network , data is supposed to be transferred to long distances.
To read more prefer those links
https://en.wikipedia.org/wiki/Ethernet_flow_control
http://virtualthreads.blogspot.in/2006/02/beware-ethernet-flow-control.html
