supply from one branch to another

You have to register for kolkata shop.......you have to send invoice charging gst with way bill........

आपको अलग अलग राज्य में अलग-अलग जीएसटीएन प्राप्त करना होगा तथा कोलकाता के लिए आपको आईजीएसटी चार्ज कर अपनी ही फर्म को बिल जारी करेंगे जिसकी आईटीसी आप क्लेम कर सकते हैं

No you can't sell goods with same GST number
You need to get a separate GST registration in West Bengal.
Transfer of goods will be treated as supply and you will get ITC for same.

First of all , Registered the shop in Kolkata , as per you it's branch of UP manufacturing Unit. (means under the same PAN)
.
As per section 7 CGST Act.
You will Charge IGST when U supply your goods from UP to Kolkota Branch shop

You have to take a separate Registration for your Kolkata Shop. GST is a Destination based Consumption Tax. Under GST, Registration is State wise. Your UP Registration will not be applicable in WB. Hence , first register your Kolkata Shop and then send goods from UP , charging IGST at the applicable rate and then claim INPUT TAX CREDIT ( ITC ) for the same.

Under GST, the Taxable Event is "Supply of GOODS or SERVICES or both". Therefore, GST will also be levied for stock transfer between branches.