java.lang.IllegalArgumentException: Children: duplicate children added: parent = Grid hgap=0.0, vgap=0.0, alignment=TOP_LEFT
    at javafx.scene.Parent$2.onProposedChange (Parent.java:454)
    at com.sun.javafx.collections.VetoableListDecorator.add (VetoableListDecorator.java:206)
    at javafx.scene.layout.GridPane.add (GridPane.java:965)
    at cs1302.arcade.Tetris.newTetro (Tetris.java:275)
    at cs1302.arcade.Tetris.play (Tetris.java:131)
    at cs1302.arcade.Tetris.moveDown (Tetris.java:321)
    at cs1302.arcade.Tetris.lambda$makeTetris$1 (Tetris.java:117)
    at com.sun.scenario.animation.shared.TimelineClipCore.visitKeyFrame (TimelineClipCore.java:239)
    at com.sun.scenario.animation.shared.TimelineClipCore.playTo (TimelineClipCore.java:180)
    at javafx.animation.Timeline.impl_playTo (Timeline.java:176)
    at javafx.animation.AnimationAccessorImpl.playTo (AnimationAccessorImpl.java:39)
    at com.sun.scenario.animation.shared.InfiniteClipEnvelope.timePulse (InfiniteClipEnvelope.java:110)
    at javafx.animation.Animation.impl_timePulse (Animation.java:1102)
    at javafx.animation.Animation$1.lambda$timePulse$25 (Animation.java:186)
    at java.security.AccessController.doPrivileged (Native Method)
    at javafx.animation.Animation$1.timePulse (Animation.java:185)
    at com.sun.scenario.animation.AbstractMasterTimer.timePulseImpl (AbstractMasterTimer.java:344)
    at com.sun.scenario.animation.AbstractMasterTimer$MainLoop.run (AbstractMasterTimer.java:267)
    at com.sun.javafx.tk.quantum.QuantumToolkit.pulse (QuantumToolkit.java:514)
    at com.sun.javafx.tk.quantum.QuantumToolkit.pulse (QuantumToolkit.java:498)
    at com.sun.javafx.tk.quantum.QuantumToolkit.pulseFromQueue (QuantumToolkit.java:491)
    at com.sun.javafx.tk.quantum.QuantumToolkit.lambda$runToolkit$403 (QuantumToolkit.java:319)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run (InvokeLaterDispatcher.java:95)
    at com.sun.glass.ui.gtk.GtkApplication._runLoop (Native Method)
    at com.sun.glass.ui.gtk.GtkApplication.lambda$null$48 (GtkApplication.java:139)
    at java.lang.Thread.run (Thread.java:748)