Q:

Advantage to running SELECT DISTINCT to SELECT

So I am fairly new to SQL and have been tasked with revamping/documenting programs at my current internship. 
To cut to the chase, I am running the following query on a table:
SELECT COUNT(*) as NumofAccounts 
    FROM Accounts 
        WHERE Deleted = 0 
        AND Account_ID In ( SELECT DISTINCT Account_ID FROM Tasks WHERE Deleted = 0 );

which yields a result of NumofAccounts = 18. 
However the original query was:
SELECT COUNT(*) as NumofAccounts 
        FROM Accounts 
            WHERE Deleted = 0 
            AND Account_ID In ( SELECT Account_ID FROM Tasks WHERE Deleted = 0 );

Both of these queries yield NumofAccounts = 18, however, if one isolates this part of the query: 
SELECT Account_ID FROM Tasks WHERE Deleted = 0; 

There are 736 rows of results, many of which are duplicate Account_IDs.
But if one adds DISTINCT: 
SELECT DISTINCT Account_ID FROM Tasks WHERE Deleted = 0; 

There are only 18 rows of results, which jibes with the NumofAccounts.
Is there any way of analyzing the runtime or efficiency discrepancy between these two queries? 
Sorry for the long-winded post!

A:

You can turn on query execution plan and look how was your query executing:

Also, you can turn on client statistics and get total execution time:

So, using these instruments you can compare different queries, and select better one.

