Fix bugs: Check duplicate task

[NereusWB922]

• Check duplicate task for bulk task assignment
• Allow assign task for all students under root

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[codecov]

Codecov Report

Merging #193 (b4aed85) into master (3d069be) will decrease coverage by 1.12%.
Report is 1 commits behind head on master.
The diff coverage is 41.25%.

@@             Coverage Diff              @@
##             master     #193      +/-   ##
============================================
- Coverage     55.37%   54.26%   -1.12%     
- Complexity      577      592      +15     
============================================
  Files           118      119       +1     
  Lines          2707     2779      +72     
  Branches        308      336      +28     
============================================
+ Hits           1499     1508       +9     
- Misses         1036     1075      +39     
- Partials        172      196      +24     

Files	Coverage Δ
src/main/java/seedu/address/logic/Messages.java	78.26% <ø> (ø)
...in/java/seedu/address/logic/commands/Category.java	100.00% <100.00%> (ø)
...ress/logic/parser/CreateDeadlineCommandParser.java	73.68% <100.00%> (ø)
.../address/logic/parser/CreateTodoCommandParser.java	72.22% <100.00%> (ø)
...in/java/seedu/address/logic/parser/ParserUtil.java	67.60% <20.00%> (-1.96%)	⬇️
.../address/logic/commands/CreateDeadlineCommand.java	50.76% <35.89%> (-25.83%)	⬇️
...eedu/address/logic/commands/CreateTodoCommand.java	52.38% <39.02%> (-31.34%)	⬇️
.../main/java/seedu/address/model/ChildOperation.java	52.87% <40.00%> (-6.78%)	⬇️

... and 1 file with indirect coverage changes

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[NgChunMan]

LGTM. It is essential to ensure there are no tasks added twice to a student.