Let me add two additional methods to the answer by @Harun24HR. Both options assume you don't have headers as per your sample data.
Option 1) : Dynamic Array Functions
When one has access to dynamic array functions you may use the following:
In C1:
=UNIQUE(A1:A17)
This UNIQUE function will spill an array of unique values from defined range into column C.
In D1:
=TEXTJOIN(",",TRUE,FILTER(B$1:B$17,A$1:A$17=C1))
Whereas FILTER will extract all values from column B where column A matches it is TEXTJOIN that will concatenate these values into your desired string.
Drag down...
Or, in a single go, exploiting TOCOL():
=LET(x,UNIQUE(TOCOL(A:A,1)),HSTACK(x,MAP(x,LAMBDA(y,TEXTJOIN(",",,FILTER(B:B,A:A=y))))))
Option 2) : PowerQuery
Would you want to experiment with PowerQuery/GetAndTransform then you don't need any formulas nor VBA for that matter. Follow these steps:
- Select 
A1:B17 and from the ribbon choose Data > From Table/Range under "Get & Transform Data" 
- Choose to import data without headers. A new window will open.
 
- From the ribbon click 
Transform > Group By. Within that menu choose to group by Column1, choose a new column name, e.g.: "Grouped" and then choose All Rows from the Operation dropdown and click OK. 
- You'll notice an extra column. Now on the ribbon click 
Add Column > Custom Column and enter the following formula: Table.Column([Grouped], "Column2"). This should add a third column that holds a list of values. 
- Remove 
Grouped from the table. Then click on the icon to the right of the newly added column name, and you'll have two options. Choose Extract Values, then choose a comma as your delimiter. 
There might be a translation-error in the M-code below, but this should be it:
let
    Source = Excel.CurrentWorkbook(){[Name="Tabel1"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", Int64.Type}, {"Column2", Int64.Type}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Column1"}, {{"Grouped", each _, type table [Column1=number, Column2=number]}}),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows", "Custom", each Table.Column([Grouped], "Column2")),
    #"Extracted Values" = Table.TransformColumns(#"Added Custom", {"Custom", each Text.Combine(List.Transform(_, Text.From), ","), type text}),
    #"Removed Columns" = Table.RemoveColumns(#"Extracted Values",{"Grouped"})
in
    #"Removed Columns"
PowerQuery is available from Excel-2010 if I'm not mistaken so you wouldn't need access to advanced formulas like TEXTJOIN to perform this.