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;