PHP, (削除) 58 (削除ここまで) 56 bytes

function f($s){return!$s?:$s[0]==$s[1]&f(substr($s,2));}

Try it online!

As a recursive function.

PHP, (削除) 61 (削除ここまで) 56 bytes

foreach(str_split($argn,2)as$a)$r|=$a[0]!=$a[1];echo!$r;

Try it online!

Or standalone program. Input string via STDIN, output is truthy (1) if it is double speak, and falsey (0) if it is not double speak.

PHP, (削除) 58 (削除ここまで) 56 bytes

function f($s){return!$s?:$s[0]==$s[1]&f(substr($s,2));}

Try it online!

As a recursive function.

PHP, (削除) 61 (削除ここまで) (削除) 56 (削除ここまで) 52 bytes

while(''<$l=$argn[$i++])$r|=$l!=$argn[$i++];echo!$r;

Try it online!

Or standalone program. Input string via STDIN, output is truthy (1) if it is double speak, and falsey (0) if it is not double speak.

-4 bytes thx to @Night2 !

PHP, (削除) 61 (削除ここまで) 56 bytes

foreach(str_split($argn,2)as$a)$r|=$a[0]!=$a[1];echo!$r;

Try it online!

Input string via STDIN, output is truthy (1) if it is double speak, and falsey (0) if it is not double speak.

PHP , 58 bytes

Or recursive:

function f($s){return$s?$s[0]==$s[1]?f(substr($s,2)):0:1;}

Try it online!

PHP, (削除) 58 (削除ここまで) 56 bytes

function f($s){return!$s?:$s[0]==$s[1]&f(substr($s,2));}

Try it online!

As a recursive function.

PHP , (削除) 61 (削除ここまで) 56 bytes

foreach(str_split($argn,2)as$a)$r|=$a[0]!=$a[1];echo!$r;

Try it online!

Or standalone program. Input string via STDIN, output is truthy (1) if it is double speak, and falsey (0) if it is not double speak.

PHP, (削除) 61 (削除ここまで) 56 bytes

foreach(str_split($argn,2)as$a)$r|=$a[0]!=$a[1];echo!$r;

Try it online!

Input string via STDIN, output is truthy (1) if it is double speak, and falsey (0) if it is not double speak.

PHP, (削除) 61 (削除ここまで) 56 bytes

foreach(str_split($argn,2)as$a)$r|=$a[0]!=$a[1];echo!$r;

Try it online!

Input string via STDIN, output is truthy (1) if it is double speak, and falsey (0) if it is not double speak.

PHP , 58 bytes

Or recursive:

function f($s){return$s?$s[0]==$s[1]?f(substr($s,2)):0:1;}

Try it online!