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You are actually confusing the actual component code which is LM139 with the component identifier on the schematic.
U1,U2,U3 etc represent one chip each and the letters A,B,C,D represent the four comparators included in each chip.
A B C D are nothing but parts per package for that device. So LM139 can be seen as collection of 4 identical devices (comparators) and they are uniquely identified as A B C D.
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