Admin should be able to add multiple containers which are identical

**[Original report](https://bitbucket.org/laborstudents/chemical-inventory-new/issue/23) by Raunak Thakur (Bitbucket: [thakurr](https://bitbucket.org/thakurr), GitHub: [thakurr](https://github.com/thakurr)).**

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There are times when identical containers of same chemical are shipped. so we need to add a functionality that lets admin add multiple containers at the same time.

Start from the admin workflow for creating chemical containers and trace how a single container is stored. Define how an admin enters the quantity and how identical containers are represented, then verify that adding multiple containers creates the expected records without changing their shared details.

Written by the indexing model from the issue text.