To reproduce:
Here is my snippet
$$\mathbb{M}\mathbf{p}^{(i)}\approx \int_{B(\mathbf{r}_i,r_{\mathrm{cut}})}\rho_{\mathrm{free}}(\mathbf{r})\mathrm{d}\mathbf{r} \tag{1}$$
$$\mathbb{M}\mathbf{p}^{(i)}\approx \int_{B(\mathbf{r}_i,r_{\mathrm{cut}})}\rho_{\mathrm{free}}(\mathbf{r})\mathrm{d}\mathbf{r} \tag{2} \label{eqtn1}$$
Refer polarisation equation $\eqref{eq1}$.
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My conclusion:
It seems to be a problem with using \label tag in equation? Can anyone help me with a way out?