FFT of a signal with 0 added between samples
You can use expansion analysis to deduce what happens when a sequence is zero staffed in between its samples.

Let $x[n]$ be your original length $N$ sequence. Then its expansion by $2$ yields the new sequence $y[n]$ of length $2N$ :

$$ y[n] = \begin{cases} { x[n/2] ~~~,~~~n = 2m \\ ~~~~~ 0 ~~~,~~~ \text{otherwise} } \end{cases} $$

Then the DTFT relation between $Y(e^{j\omega})$ and $X(e^{j \omega})$ will be : $$Y(e^{j\omega}) = X(e^{j 2 \omega})$$

Then the DFT relation between N-point $X[k]$ and 2N-point $Y[k]$ will be

$$Y[k] = Y(e^{j \frac{2\pi}{2N} k}) = X(e^{ j 2 \frac{2\pi}{2N} k}) =  X[k] ~~~,~~~ k = 0,1,...,2N-1$$

In other words, $Y[k]$ will be two copies of $X[k]$