Is there any advantage or disadvantage to add subdomain twice in search engines?

Question

I have a domain (example.com) and some subdomains (e.g., sd1.example.com, ...). I added example.com to Google and Bing Webmaster Tools. I added example.com/sitemap and sd1.example.com/sitemap (with a redirect) for this site on Webmaster Tools.
Then I added sd1.example.com as a site in Webmaster Tools and added sd1.example.com/sitemap for this site. I did this for all subdomains.

1) Now, is there any benefit for adding subdomains and their sitemaps twice? Or are there any disadvantages?

2) Which one is better? (from an SEO point-of-view)

3) If I want to change example.com domain to newdomain.com, should I redirect URLs separately in Webmaster Tools?

Edit:
Clarifying the above questions:

1) For example, is there any penalty from Google in adding a sitemap twice in two websites? Does Google show two results with the same URL (because I submitted a URL twice)?

2) Is it better to have one indexed result for same URL or two?

3) After moving example.com to newdomain.com in Webmaster Tools, is there any need to do it for subdomains too?

Edit:
I need to have separate Webmaster Tools sites for subdomains. Before, I added sitemaps to both main domain and subdomain in Webmaster Tools. Now, that I have changed main domain, I must remove old subdomains from Webmaster Tools. In this situation sitemaps vanish and I do need to submit sitemap for new subdomains. Will doing this cause Google to lose data for my subdomains?

Answer 1

1. I don't think there is a penalty but there is no benefit and it may cause problems. Why do something unnecessary if you don't have to?. Since the subdomains have their own GWT accounts submitting a sitemap for each subdomain would be fine.

2. Once a page is indexed it will be displayed no more than once for any given search term but can rank for an unlimited number of search terms.

3. Yes. Every page, even those on subdomains, must do a 301 redirect to their new URL or else they will be considered new pages and all SEO value they have (i.e. incoming links) will be lost. Making new sitemaps would be needed as well.