Yes, it is generally possible to use the air filter from a 2016 KTM Duke 390 in a 2018 KTM Duke 390. The KTM Duke 390 models from 2013 to 2021 have a high degree of parts compatibility, including the air filter. The air filter for these models typically has the same dimensions and specifications, allowing for interchangeability.
If you have access to both air filters, you can compare them side by side to check if they have identical dimensions and configurations. Ensuring a proper fit and compatibility is crucial for the air filter's performance and overall engine health.