[FIX] some translation terms not imported

Stricter filter criteria to detect duplicates translations. Do not skip translations for the same terms but belonging in different modules.
Fixes #4239

**Step to reproduce issue:**
 1. install website_sale module
 2. install a language with enough terms (e.g. ru_RU), available on public website
 3. check the checkout view on e-shop

**Results:**
Title of cart lines is "Продукт - Price - Quantity"

**Expected:**
Title of cart lines is (of course) "Продукт - Цена - Количество"

**Reason of the error**
module sale has already a term Quantity [in sale.pot](https://github.com/odoo/odoo/blob/00c234bc8582b10e1dcd892d6563c948bf9e1808/addons/sale/i18n/sale.pot#L1375) and ignored during import.

Fix made with @rco-odoo.
 @odony could you check if ok for you?