Fix modulation filter to check for multiple entries separated by

semicolon. Also add a check to remove duplicated signals when
loading the database from file

constants.py

@@ -176,6 +176,7 @@ class Constants:
     EXTRACTING_CODE         = -1
     NOT_AVAILABLE           = "spectrumnotavailable.png"
     NOT_SELECTED            = "nosignalselected.png"
+    FIELD_SEPARATOR         = ";"
     DEFAULT_IMGS_FOLDER     = os.path.join(":", "pics", "default_pics")
     DEFAULT_NOT_SELECTED    = os.path.join(DEFAULT_IMGS_FOLDER, NOT_SELECTED)
     DEFAULT_NOT_AVAILABLE   = os.path.join(DEFAULT_IMGS_FOLDER, NOT_AVAILABLE)