ICAO recommendations include a protection for same-frequency navaids, restricting the received power of undesired stations to a level preventing interferences. To meet this restriction same-frequency stations must be considerably distant.
The result, in normal conditions, is the receiver cannot switch uninterrupted from the current station to a same-frequency station, there will be a significant period where none of the stations can be received.
However distant stations signals can be significantly increased by propagation perturbations like sporadic E propagation or wave ducting. These instable conditions can break the co-channel protection plan and increase distant stations levels above the reception threshold, possibly above the local station level. When this occurs the receiver can wander between stations unnoticed if the station ID is not checked.
I'll detail the answer for VHF navaids, but it can be extrapolated to other navaids.
ICAO co-channel protection
Are there two navaids on the same frequency close enough to be confused?
The normal range for VHF is the radio horizon, that is 15% further than the geometrical horizon (k-factor = 4/3).
Same-frequency navaids are spaced well further than this horizon and by measuring the signal level rather than the distance. Using the signal level allows to take into account directional transmitters, like ILS components, where energy is concentrated in some solid angular sector.
At any point within the service volume of the desired VOR, the ratio between the reception level of the VOR and the reception level of any same-frequency VOR must be greater than the co-channel protection factor:
Co-channel interference for a VOR, source
This ratio is 20dB (= 100 times) for ILS/VOR, and 15dB (32 times) for NDB. More information in ICAO Doc 9718, chapter 9. Note this ratio is the recommended minimum, but if for some reason this ratio is not effective, then a solution must be found, ICAO/ITU have procedures to work exceptions.
When the desired VOR transmits and is within range, the receiver reduces its amplification to the bare minimum and the undesired signal, given the protection factor, is undetectable. When the receiver is located at the limit of the service volume, the other signal must still be 20dB weaker. It means the two VOR cannot have their service volume next to each other, but must be significantly remote.
But there are circumstances where propagation can be boosted, and distant stations behave like if they were considerably closer they actually are.
Are there example locations where you can be just outside the range of one navaid and pick up another on the same frequency?
Some conditions of propagation allow reception of signals well beyond their usual limits of reception:
Night effect, effective under 30 MHz, may affect NDB. This perturbation is common, but its exact result is unpredictable. See What is the night effect?
Tropospheric ducting, due to a temperature inversion in troposphere, mostly applies to the lower VHF band. As refraction index varies with temperature, a tunnel is created between two air layers and the signal is ducted like light in a graded-index optical fiber.
Sporadic E, which affects VHF, is a situation where a signal is reflected by ionosphere and can be received at distance larger than 2,500 km.
In polar regions auroras are ionized curtains created by solar winds which alter propagation in unpredictable ways.
The actual use of the frequency, navigation, data, voice, has no significant impact.
Consequences regarding co-channel protection
With this in mind, the coverage of a navaid is not limited by a circle of some radius, you can be at a location where two navaids at very different distances are received with an equal power, or where the remote navaid is stronger than the local one.
Such super-propagation events are more frequent in HF band, and the night effect is present each night. VHF/UHF events are seen during short periods of time, between 10 and 30 minutes a few times per month, more often around summer solstice.
ID check is required before signal use
Communication cannot be protected against these effects. The possibility to receive a remote navaid stronger than a local one can never been eliminated, even with a careful planning of the radio services and ground stations.
That's why when lives are at stake, identifying the actual navaid received is mandatory. Note the protection factors listed at the beginning already include a 6dB (4 times) margin specific to aeronautical communications, not used for common spectrum planning.