identical

adj. exactly alike, same; equal in every respect, interchangeable

In quantum mechanics, identity of particles of the same kind (say, electrons) mathematically results in profound consequences for their collective behavior - quantum statistics of bosons and fermions.
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Adjective
1. exactly alike; incapable of being perceived as different; "rows of identical houses"; "cars identical except for their license plates"; "they wore indistinguishable hats"
(synonym) indistinguishable
(similar) same
2. being the exact same one; not any other:; "this is the identical room we stayed in before"; "the themes of his stories are one and the same"; "saw the selfsame quotation in two newspapers"; "on this very spot"; "the very thing he said yesterday"; "the very man I want to see"
(synonym) one and the same(p), selfsame(a), very(a)
(similar) same
3. (of twins) derived from a single egg or ovum; "identical twins are monovular"
(synonym) monovular
(antonym) fraternal, biovular
4. having properties with uniform values along all axes
(similar) isotropic, isotropous
(classification) physics, physical science, natural philosophy
5. coinciding exactly when superimposed; "identical triangles"
(synonym) superposable
(similar) congruent

(a.)
Uttering sameness or the same truth; expressing in the predicate what is given, or obviously implied, in the subject; tautological.
  

(a.)
The same; the selfsame; the very same; not different; as, the identical person or thing.
  

qian1 ren2 yi2 mian4
Thousand persons with one face.
All look the same.

nan4 xiong1 nan4 di4
Suffering big brother, suffering little brother.
Originally meaning brothers with the same ability and virtue. Later describing people who are in the same boat, struggling in difficulties together. Two of a kind.