I claim the ornamental design for a dual flush button, as shown and described.
FIG. 1 is a top perspective view of a dual flush button in accordance with the present invention;
FIG. 2 is a top elevation view thereof;
FIG. 3 is a bottom elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is front elevational view thereof; and,
FIG. 7 is a rear elevational view thereof.
The broken line showing back portions of the dual flush button is included for the purpose of illustrating a portion of the dual flush button that forms no part of the claimed design.