Remove duplicates + concatenate data

Your profile shows that you use Office 365. I think TEXTJOIN function should work for you
To get unique ID list
Enter in D2 and pull formula across to E2 and copy down

Formula:

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then use this array formula:
Enter in F2 and copy down

Formula:

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**Array formula must be entered with Ctrl+Shift+Enter key combination.

v	A	B	C	D	E	F
1
2	ID1	John Smith	Skill1	ID1	John Smith	ID1 John Smith Skill1; Skill2; Skill3
3	ID1	John Smith	Skill2	ID2	Elvis Presley	ID2 Elvis Presley Skill3; Skill4
4	ID1	John Smith	Skill3	ID3	Susan Boyle	ID3 Susan Boyle Skill1; Skill6
5	ID2	Elvis Presley	Skill3
6	ID2	Elvis Presley	Skill4
7	ID3	Susan Boyle	Skill1
8	ID3	Susan Boyle	Skill6

You can get similar results with a pivot table. It probably depends on how you want to look at the data.