Formula HELP! SUM of Duplicate entries

[RobynA]

I have this spreadsheet that I get at work - and the original is 45,000+ lines so I need a formula to help me from doing it manually. I will paste a snippet below.
Basically this is the issue:
Vessel name changes but date range is from April 1, 2019 to March 31, 2020 so all vessels have similar dates so I can't do a straight duplicate formula to calculate one particular vessel and highlight when it has April 15th more than once, because it'll highlight all April 15th dates (for all vessels).

I need it to give me a SUM of the total duplicate days for each vessel. I tried a countIF and a SUMIF together but it's not working properly.

For example: Vessel 1 April 1 - days 1 but on April 15th there are two inputs one like 0.5 and another april 15th with 0.7 I want them to add up.

 

[RobynA]

I should also note, that I tried subtotal and in theory would work but it makes the spreadsheet way too long to go through (doubles in size) and crashes my computer when I try to apply it. Thanks for any help!!