[werk_ag]

You can either hermetically seal them into a waterproof box, or put the pcbs into a vented enclosure with the vent being a breathing dessicator, expensive though!

Sharing my experience...
I'm using a big waterproof case, but this case is not sealed, it have a small hole on the bottom. I never saw any sign of condensation inside, and I live in a place where air moisture levels, specially during winter, are usually high.
A small case is more prone to condensation than a bigger one.

My transmitter unit is here since 3 years, working as new. As seen on the picture, my TX unit even is one of the older versions which don't have an on-board SMA connector for the antenna.

This transmitter unit is only sending data of my wind instruments, all the other sensors (temp / hum,  rain gauge, solar Rad / UV) are connected to another transmitter unit.

[qldbureau]

The problem is active right now on mine after a couple of weeks trouble free.  As is the norm I'm either at work as is the present, or it is overnight while I'm sleeping.

I recently finished my Pro Plus build (more on that another time) which I am running receiving the same signal as my live Pro 2 as a test.  I noticed this morning a discrepancy between displays, in rain fall and marginal difference on outside humidity. (All from the same sensors, no individual sets)

I will download the Pro 2 Plus log and see that it is not a reception issue on the Pro 2.

[werk_ag]

I noticed this morning a discrepancy between displays, in rain fall ...

In two specific conditions, even with a perfectly working system, two receivers getting data from the same transmitter unit, can show different Today Rain values.
Imagine this situation:

It is raining during your meteorological day rollover time (for our example it happens at midnight).
At 23:59:59 of day X, your TX unit sends a data packet with Rain info.

Condition 1 - Supposing that the RTC time of both receivers are perfectly in sync (very unlikely).

Receiver 1 receives the packet, and updates the Today Rain for day X, next it will do the meteorological rollover and starts day Y with Today Rain at zero.
Receiver 2 don't receive that data packet and only gets the next data packet after day rollover (because it is busy doing other things like updating the screen, etc, etc), so it ends day X with a Today Rain lower than Receiver unit 1, and starts day Y with a Today Rain greater than receiver 1.

Condition 2 - The RTC time of both receivers are not perfectly in sync (very likely).

Even if both units have received the same data packet, a difference of just a second on the RTC time of both units, will cause the same effect described above. One ends day X with a lower Today Rain than the other, and will start day Y with a higher value.

In reality, in a perfectly working system (good reception conditions) this situation does not imply a major problem on the records, because generally the difference isn't greater than just the value of the tipping bucket of your rain gauge. As all on life, the better it is, the lower the value of the tipping bucket, and lesser the possible error.

When you are running to receivers side by side, the described situation can happen with any kind or brand of weather station, its not an exclusive of the WeatherDuino Pro2 system