ECM modules currently stack for combat purposes. The design screen does not show the increased ECM rating but it does show in the combat reports in the event log.
Steps to reproduce:
Design a ship and add multiple ECM modules.
Spawn in the ship and spawn in another ship to shoot at it.
Fire at the ship with multiple ECM modules. Note that your chance to hit has been reduced by each of the ECM modules, not just by one.
This means that, for example, a ship with 20 ECM 2 modules reduces incoming fire by 400%, making it effectively immune to everything.
Example logs from firing at ships with 1, 2 and 3 ECM 3 modules:
Sensor data reveals that the alien class AMT Target ECM 3 x1 has an ECM rating of 30
Sensor data reveals that the alien class AMT Target ECM 3 x2 has an ECM rating of 60
Sensor data reveals that the alien class AMT Target ECM 3 x3 has an ECM rating of 90
AMT Tribal 001 attacked AMT Target ECM 3 x1 001 with Single 25.0cm C4 Far Ultraviolet Laser Turret. Range 0 km Chance to Hit 100% Shots 1 Damage per Hit 16 Armour Hits 1 New Target Speed 1 km/s
AMT Target ECM 3 x1 001 attacked by AMT Tribal 001 using energy weapons. Range 0 km Shots 1 Damage per Hit 16 Armour Hits 1
AMT Tribal 001 attacked AMT Target ECM 3 x2 001 with Single 25.0cm C4 Far Ultraviolet Laser Turret. Range 0 km Chance to Hit 70% Shots 1 Damage per Hit 16 Armour Hits 1 New Target Speed 1 km/s
AMT Target ECM 3 x2 001 attacked by AMT Tribal 001 using energy weapons. Range 0 km Shots 1 Damage per Hit 16 Armour Hits 1
AMT Tribal 001 attacked AMT Target ECM 3 x3 001 with Single 25.0cm C4 Far Ultraviolet Laser Turret. Range 0 km Chance to Hit 40% Shots 1 Damage per Hit 16 Armour Hits 1 New Target Speed 1 km/s
AMT Target ECM 3 x3 001 attacked by AMT Tribal 001 using energy weapons. Range 0 km Shots 1 Damage per Hit 16 Armour Hits 1
Iceranger's been doing some further testing on this bug on the Discord but isn't able to make a post on the forum at the moment so I'm posting this here so it doesn't get missed - he should be along later with more details of his testing.