Check duplicate entry

[lctxct]

Closes https://github.com/AY2324S1-CS2113T-W11-2/tp/issues/159. Yeah, I'm still not sure about this, but here are the changes...

Modifications Overall description: Make sure that tuple (date, species, name) is unique in EntryList (if possible, it would be better to change from using ArrayList to HashMap, for enforcement of unique keys).

• Modified AddCommandParser to include additional check that entry is not a duplicate
• Modified FileLoader to make sure that no duplicate entries exist in the save file
• Modified EditCommandParser to include check that the (date, species, name) of the changed entry does not overlap with an existing entry

[woodenclock]

Just realised, I can only test it out after merging :)

[woodenclock]

Upon testing, looks good