I dont understand by "acting kind of weird"... Follow this, 1. Align the model with the axis.Because u wanna mirror with respect to it. 2. Select the move tool 3. Press the INSERT button. 4. Snap the Pivot point with the axis by pressing x and mouse middle button(this button is to snap with grid) 5. go to edit>Duplicate special 6. reset settings 7. Now figure out on which direction u wanna reflect the object.If u wanna reflect the object on negative x, then add a negative sign before the 1 in the next box of "Scale".For y axis go to the 2nd box and for z axis use the last one. 7. Press Duplicate Special. If u still face problem try deleting the history before duplicating.