When ENCs, of the same banding, from differing producing nations, overlap, the ECDIS has to decide which ENC to display. It should also display an 'Overlapping Charts' message to say that there is overlapping data.
The ECDIS should then ONLY display the data from its chosen ENC. If there is a doubling of data or an issue with the display, then this would be down to the ECDIS and how it is choosing to display the data.
Therefore unless there is an issue with the ECDIS displaying the data, the 'Overlapping Charts' message being displayed is correct.
Bandings are as follows:
See the attached screen shots as an example.