To answer your first question, Susan; no, you don't need it twice. It shouldn't even need topping up. (How could you tell?)
For your second question: who do you trust? Where there are "soft targets". there will always be chancers, a-holes, and "consultants".
A simple rule, worth following, is to reject anything unsolicited. If someone wants a product or service, then they should be pro-active. In other words, ask around. Recommendations, local knowledge etc.
Insulation installation is generally low-tech and straightforward. There are plenty of competent installers out there. Lists of approved contractors in this industry usually consist of anyone who can pay the registration fee. It is no guarantee.
This sort of thing will always happen, as long as vulnerable people continue to let them in the door.