Wrong display two numbers at the same time on different digits.

What model of clock and what kind of tubes?

If the clock has been working fine for a while and then this problem appeared I can only think of 2 possible causes:

1) One of the tubes has developed an internal short between the 2 and 3 cathodes.

2) The K155ID1 (74141) has partially failed.

These internal shorts seem to happen fairly often with some models of Russian tubes. In the case of the IN-14 a shorted 2-3 isn't too surprising because those digits are next to each other in the digit "stack".

The 0 digits on all 6 tubes are tied together, the 1s all tied together, the 2s all tied together ... and so on. Standard multiplex design. So, if two digits short together in any tube it has the effect of shorting those digits for all tubes. Or to say it another way, if one tube has the 2 & 3 shorted, that shorts the "2 line" to the "3 line". The result is that any time any tube - not just the shorted one - is supposed to display a 2 or a 3 both digits will light up.

I hope that makes sense. I found it rather hard to explain clearly in words.

Here is a page from the Classic kit manual that clearly shows how the digits are all tied together. The Modular's tube connections are electrically the same but it's not as clear on the Modular schematic as it is in the Classic. It's shown in detail in the Classic manual because the builder has to wire up all the tube connections by hand whereas on the Modular it's all on the PCB where the builder doesn't have to worry about it.