Assume your data (with headers) looks like this:
Qty
Lib1
An 100A
An 101B
An 102C
An 103D
An 104E
Bn 105F
Cn 106G
Cn 107H
Cn 108J
Dn 109K

Create the Lib2 list of unique items in Lib1 with
Data Filter Advanced Filter Copy to another location
Unique Records only

Lib2
An 104E 103D 102C 101B 100A
Bn 105F
Cn 108J 107H 106G
Dn 109K

Name Lib1, Lib2, and Qty.
The first cell of Qty contains a space.
All formulas are in R1C1 ref style.
Create these addittional names:
bins Refers To: =ROW(INDEX(C1,1):INDEX(C1,ROWS(Lib1)))
reps Refers To: =MAX(COUNTIF(Lib1,Lib1))
colu Refers To: =COLUMN(INDEX(R1,1):INDEX(R1,reps))
Select the 5 cells next to Lib2, An
(5=reps=max repetitions in Lib1)
and enter this array formula:
=INDEX(Qty,LARGE(TRANSPOSE(IF(Lib1=Lib2 R,bins,0)),colu)+1)
With the fill handle, drag this set of cells down to fill the array.
Select A1 ref style to translate all to your preference.