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2 -1 bytes by printing directly instead of returning
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Java 8, 93 9191 90 bytes

v->{String r="",t=r;fort="";for(char c=64;++c<91;t+=c)r+=rSystem.out.println(t.join(c+"",t.split(""))+c+"\n";return r;+c);}

-1 byte thanks to @OlivierGrégoire by printing directly instead of returning

Explanation:

Try it online.Try it online.

v->{                     // Method with empty unused parameter and String return-type
  String r="",t=r;t="";       //  Result and temp// Strings, bothTemp-String, starting empty
  for(char c=64;++c<91;  //  Loop over the letters of the alphabet:
      t+=c)              //    After every iteration: append the letter to the temp-String
    r+=                System.out.println(  //   AddPrint towith thetrailing resultnew-Stringline:
       r.join(c+"",t.split(""))
                         //    The temp-String with the current letter as delimiter
       +c   );}             //    + the current letter as trailing character 
       +"\n";            //    + a new-line
  return r;}             //  After the loop: return the result (with trailing new-line)

Java 8, 93 91 bytes

v->{String r="",t=r;for(char c=64;++c<91;t+=c)r+=r.join(c+"",t.split(""))+c+"\n";return r;}

Explanation:

Try it online.

v->{                     // Method with empty unused parameter and String return-type
  String r="",t=r;       //  Result and temp Strings, both starting empty
  for(char c=64;++c<91;  //  Loop over the alphabet:
      t+=c)              //    After every iteration: append the letter to the temp-String
    r+=                  //   Add to the result-String:
       r.join(c+"",t.split(""))
                         //    The temp-String with the current letter as delimiter
       +c                //    + the current letter as trailing character 
       +"\n";            //    + a new-line
  return r;}             //  After the loop: return the result (with trailing new-line)

Java 8, 93 91 90 bytes

v->{String t="";for(char c=64;++c<91;t+=c)System.out.println(t.join(c+"",t.split(""))+c);}

-1 byte thanks to @OlivierGrégoire by printing directly instead of returning

Explanation:

Try it online.

v->{                     // Method with empty unused parameter and String return-type
  String t="";           //  Temp-String, starting empty
  for(char c=64;++c<91;  //  Loop over the letters of the alphabet:
      t+=c)              //    After every iteration: append the letter to the temp-String
    System.out.println(  //   Print with trailing new-line:
       r.join(c+"",t.split(""))
                         //    The temp-String with the current letter as delimiter
       +c);}             //    + the current letter as trailing character 
1
source | link

Java 8, 93 91 bytes

v->{String r="",t=r;for(char c=64;++c<91;t+=c)r+=r.join(c+"",t.split(""))+c+"\n";return r;}

Explanation:

Try it online.

v->{                     // Method with empty unused parameter and String return-type
  String r="",t=r;       //  Result and temp Strings, both starting empty
  for(char c=64;++c<91;  //  Loop over the alphabet:
      t+=c)              //    After every iteration: append the letter to the temp-String
    r+=                  //   Add to the result-String:
       r.join(c+"",t.split(""))
                         //    The temp-String with the current letter as delimiter
       +c                //    + the current letter as trailing character 
       +"\n";            //    + a new-line
  return r;}             //  After the loop: return the result (with trailing new-line)