PHP, 58 56 bytes

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

Try it online!

As a recursive function.

PHP, 61 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, 58 56 bytes

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

Try it online!

As a recursive function.

PHP, 61 56 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, 61 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, 58 56 bytes

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

Try it online!

As a recursive function.

PHP, 61 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, 61 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, 61 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!