k2 v2.6.9 for j25 --- Wrong count of duplicate aliases
administrator\components\com_k2\tables\k2item.php
--- Wrong ( 0 not 1... and lost bracket in 181)
179 if ($result > 1)
181 $this->alias .= '-'.(int)$result + 1;
--- Correct
179 if ($result > 0)
181 $this->alias .= '-'.((int)$result + 1);

administrator\components\com_k2\tables\k2category.php
--- The same thing on rows 193, 195

You are wellcome... :)

Fixed with github.com/joomlaworks/k2/commit/12671f8088a90f44ef8e2377334768bdc5139670 .

Thanks for reporting.