Effect in Fourier-domain when the signal is "zero-stuffed" in time-domain
Let's be more general and consider the insertion of $L-1$ zeroes between each sample. Mathematically, this can be written as
$$
y[m]=
\begin{cases}
x[n] &amp; \text{if } m = nL\\
0  &amp; \text{otherwise}
\end{cases}.
$$
The DTFT of $y[n]$ is then given by
$$Y(e^{j\Omega}) = \sum_{m=-\infty}^{\infty} y[m]e^{-jm\Omega}.$$
Because $y[m]$ is non-zero only when $m$ is a multiple of $L$, that is $m = nL$, this becomes
$$Y(e^{j\Omega}) = \sum_{n=-\infty}^{\infty} x[n]e^{-jnL\Omega} = X(e^{jL\Omega}).$$
From this, you can see that the DTFT of $y$ is the DTFT of $x$ with a period $L$ times smaller (i.e. $2\pi/L$ instead of $2\pi$). Intuitively, this makes sense: you know that "expanding" a signal in the time-domain is equivalent to "squeezing" its spectrum in the frequency domain (see the time scaling property of the Fourier transform).