Recently have found that the duplicate labels options, for point features only, are all disabled. I can make changes in the "Point Settings" section, but all the options under "Duplicate Labels" in the Placement Properties dialog, are all greyed out. The placement properties are not disabled for line and polygon features. These all work okay.
From context help (click "?" besides "x", then click on parameter): Point features: No duplicate label options are available. One label is always placed per feature part.
Line features: When two or more contiguous features have the same value for the label string, only one label will be placed. This prevents unnecessary duplication of the same label. For example, in a layer representing the rivers of South America, the Amazon river may be represented by a number of contiguous line features, with each section of the river between each tributary being a separate feature. When this option is selected, the label "Amazon" will be appear just once along the entire river, even though it is made up of several features.
This is the default for line features. Polygon features: When two or more features have the same value for the label string, only one label will be placed. The features need not be contiguous.