Bugfix for add to consult command

[Clarenceeey]

Fixes #268 , partially addresses #299

Previously, when multiple of the same students are added in the same command, an exception is thrown.

Now, when that happens, the student is added successfully.

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...gic/commands/consultation/AddToConsultCommand.java	95.23%	0 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ	Complexity Δ
...gic/commands/consultation/AddToConsultCommand.java	88.88% <95.23%> (+0.55%)	16.00 <1.00> (+4.00)

... and 3 files with indirect coverage changes

[notnotmax]

lgtm, see here for argument on why it should be allowed.