Look up display before registering datasets

If there's already a display in the legacy image map for a given
dataset, then we know that dataset has already been registered and do
not need to re-register.

Registration always creates a new ImagePlus so this avoids a bug where
ImagePlus instances can proliferate.

src/main/java/net/imagej/legacy/convert/DatasetToImagePlusConverter.java

@@ -34,6 +34,8 @@
 import java.util.Collection;
 
 import net.imagej.Dataset;
+import net.imagej.display.ImageDisplay;
+import net.imagej.legacy.LegacyImageMap;
 
 import org.scijava.Priority;
 import org.scijava.convert.Converter;
@@ -66,7 +68,19 @@ public class DatasetToImagePlusConverter extends
 	public <T> T convert(final Object src, final Class<T> dest) {
 		if (!legacyEnabled()) throw new UnsupportedOperationException();
 		final Dataset d = (Dataset) src;
-		final Object imp = legacyService.getImageMap().registerDataset(d);
+		LegacyImageMap imageMap = legacyService.getImageMap();
+		Object imp = null;
+		// First see if we can find a display already showing our Dataset
+		for (ImageDisplay display : imageMap.getImageDisplays()) {
+			if (display.isDisplaying(d)) {
+				imp = imageMap.lookupImagePlus(display);
+				break;
+			}
+		}
+		if (imp == null) {
+			// No existing display so register the dataset
+			imp = imageMap.registerDataset(d);
+		}
 		@SuppressWarnings("unchecked")
 		final T typedImp = (T) imp;
 		return typedImp;