Wherre are the adjacent characters in the title? [3, 4]! Title misspelled on purpose. Read more to find out why.

Your task: given a delimited string or list including the characters A,B,C,D, output the indexes of all the adjacent equal characters. Output can be multiple strings/integers on multiple lines, a list/array, or a delimited string.

All output should be in a list or string, or multiple printed lines. Each printed line, if there are multiple, should only contain 1 string or number. Trailing whatevers are okay.

Standard methods of input/output. Standard loopholes apply.

For example, the input 'ABCDDCBA' should output 3,4 or 4,5, depending on whether it is 0- to 1- indexed, because those numbers are the indexes of D and the D next to it.

Test cases:

Test cases have input given as a single string, and output as a ,-delimited string. Outputs are 0-indexed, add 1 to every outputted item to get it to be 1-indexed.

Input: 'ABCDCABCD'
Output: ''

Input: 'AABBCCDD'
Output: '0,1,2,3,4,5,6,7'

Input: 'ABCDDDCBA'
Output: '3,4,5'

Input: 'ABBCDD'
Output: '1,2,4,5'


This is code-golf, so shortest code wins!
Retina, 33 29 23 bytes

Saved 6 bytes thanks to Martin Ender

T`L`:`(.)\1+
:
$.`¶
T`L


Outputs a linefeed-separated list of indices.

Try it online!

Explanation

T`L`:`(.)\1+


Transliterate runs of the same character into colons, to mark positions where there are duplicate characters.

:
$.`¶


Then replace each colon with the length of the text before it, followed by a linefeed.

T`L


Finally, delete any remaining letters.