Duplicate Page getting indexed and not the main page!

[Ishrat-Khan]

Main Page: www.domain.com/service
Duplicate Page: www.domain.com/products-handler.php/?cat=service

1. My page was getting indexed properly in 2015 as: www.domain.com/service
2. Redesigning done in Aug 2016, a new URL pattern surfaced for my pages with parameter "products-handler"
3. One of my product landing pages had got 301-permanent redirected on the "products-handler" page
MAIN PAGE: www.domain.com/service GETTING REDIRECTED TO: www.domain.com/products-handler.php/?cat=service
4. This redirection was appearing until Nov 2016.
5. I took over the website in 2017, the main page was getting indexed and deindexed on and off.
6. This June it suddenly started showing an index of this page "domain.com/products-handler.php/?cat=service"
7. These "products-handler.php" pages were creating sitewide internal duplicacy, hence I blocked them in robots.
8. Then my page (Main Page: www.domain.com/service) got totally off the Google index

Q1) What could be the possible reasons for the creation of these pages?
Q2) How can 301 get placed from main to duplicate URL?
Q3) When I have submitted my main URL multiple times in Search Console, why it doesn't get indexed?
Q4) How can I make Google understand that these URLs are not my preferred URLs?
Q5) How can I permanently remove these (products-handler.php) URLs?

All the suggestions and discussions are welcome! Thanks in advance!

[DonnaDuncan]

What is your domain Ishrat-Khan? I want to take a look at the live site.